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ELECTRONICS
and CIRCUIT
ANALYSIS
using MATLAB
JOHN
O. ATTIA
Department of Electrical Engineering
Prairie View A&M University
CRC Press
Boca Raton London New York Washington, D.C.
© 1999 CRC Press LLC
Library of Congress Cataloging-in-Publication Data
Attia, John Okyere.
Electronics and circuit analysis using MATLAB / John Okyere Attia
p. cm.
Includes bibliographical references and index.
ISBN 0-8493-1176-4 (alk. paper)
1. Electronics--Data processing. 2. Electric circuit analysis-Data processing. 3. MATLAB (Computer file) I Title.
TK7835.A88 1999
98-46071
621.381’0285--dc21
CIP
This book contains information obtained from authentic and highly regarded sources.
Reprinted material is quoted with permission, and sources are indicated. A wide variety of
references are listed. Reasonable efforts have been made to publish reliable data and
information, but the author and the publisher cannot assume responsibility for the validity
of all materials or for the consequences of their use.
Neither this book nor any part may be reproduced or transmitted in any form or by any
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Direct all inquiries to CRC Press LLC, 2000 Corporate Blvd. N.W. , Boca Raton, Florida
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Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation, without intent to infringe.
© 1999 by CRC Press LLC
No claim to original U.S. Government works
International Standard Book Number 0-8493-1176-4
Library of Congress Card Number 98-46071
Printed in the United States of America 1 2 3 4 5 6 7 8 9 0
Printed on acid-free paper
PREFACE
MATLAB is a numeric computation software for engineering and scientific
calculations. MATLAB is increasingly being used by students, researchers,
practicing engineers and technicians. The causes of MATLAB popularity are
legion. Among them are its iterative mode of operation, built-in functions,
simple programming, rich set of graphing facilities, possibilities for writing
additional functions, and its extensive toolboxes.
The goals of writing this book are (1) to provide the reader with simple, easy,
hands-on introduction to MATLAB; (2) to demonstrate the use of MATLAB for
solving electronics problems; (3) to show the various ways MATLAB can be
used to solve circuit analysis problems; and (4) to show the flexibility of
MATLAB for solving general engineering and scientific problems.
Audience
The book can be used by students, professional engineers and technicians. The
first part of the book can be used as a primer to MATLAB. It will be useful to
all students and professionals who want a basic introduction to MATLAB.
Parts 2 and 3 are for electrical and electrical engineering technology students and
professionals who want to use MATLAB to explore the characteristics of
semiconductor devices and the application of MATLAB for analysis and
design of electrical and electronic circuits and systems.
Organization
The book is divided into three parts: Introduction to MATLAB, Circuit analysis
applications using MATLAB, and electronics applications with MATLAB. It is
recommended that the reader work through and experiment with the examples at
a computer while reading Chapters 1, 2, and 3. The hands-on approach is one of
the best ways of learning MATLAB.
Part II consists of Chapters 4 to 8.
This part covers the applications of
MATLAB in circuit analysis. The topics covered in Part II are dc analysis,
transient analysis, alternating current analysis, and Fourier analysis. In addition,
two-port networks are covered. I have briefly covered the underlying theory and
concepts, not with the aim of writing a textbook on circuit analysis and
electronics.
Selected problems in circuit analysis have been solved using
MATLAB.
© 1999 CRC Press LLC
Part III includes Chapters 9, 10, 11 and 12. The topics discussed in this part are
diodes, semiconductor physics, operational amplifiers and transistor circuits.
Application of MATLAB for problem solving in electronics is discussed.
Extensive examples showing the use of MATLAB for solving problems in
electronics are presented.
Each chapter has its own bibliography and exercises.
Text Diskette
Since the text contains a large number of examples that illustrate electronics
and circuit analysis principles and applications with MATLAB, a diskette is
included that contains all the examples in the book. The reader can run the
examples without having to enter the commands. The examples can also be
modified to suit the needs of the reader.
Acknowledgments
I appreciate the suggestions and comments from a number of reviewers including
Dr. Murari Kejariwal, Dr. Reginald Perry, Dr. Richard Wilkins, Mr. Warsame
Ali, Mr. Anowarul Huq and Mr. John Abbey.
Their frank and positive
criticisms led to considerable improvement of this work.
I am grateful to Mr. Zhong You for typing and running some of the MATLAB
programs in this book and I am also grateful to Mr. Carl Easton and Mr. Url
Woods for drawing the circuit diagrams found in the text. I thank Ms. Debbie
Hawkins and Cheryl Wright who typed several parts of this book. I am
appreciative of Ms. Judith Hansen for her editing services. Special thanks go
Ms. Nora Konopka, at CRC Press, who took an early interest in this book and
offered me any assistance I needed to get it completed. I thank Ms. Mimi
Williams, at CRC Press, for thoroughly proofreading the manuscript.
The questions and comments from electrical engineering students at Prairie
View A&M University led to rewriting some sections of this work. Special
thanks go to the students who used various drafts of this book and provided
useful comments.
A final note of gratitude goes to my wife, Christine N. Okyere, who encouraged
me to finish the book in record time. With equanimity and understanding, she
stood by me during the endless hours I spent writing.
© 1999 CRC Press LLC
DEDICATION
Dedicated to my family members
Christine, John II and Angela
for
their unfailing love, support and encouragement
© 1999 CRC Press LLC
CONTENTS
CHAPTER ONE
1.1
1.2
1.3
1.4
1.5
1.6
MATLAB FUNDAMENTALS
MATLAB BASIC OPERATIONS
MATRIX OPERATIONS
ARRAY OPERATIONS
COMPLEX NUMBERS
THE COLON SYMBOL ( : )
M-FILES
1.6.1 Script files
1.6.2 Function files
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER TWO PLOTTING COMMANDS
2.1
2.2
2.3
2.4
GRAPH FUNCTIONS
X-Y PLOTS AND ANNOTATIONS
LOGARITHMIC AND POLAR PLOTS
SCREEN CONTROL
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER THREE CONTROL STATEMENTS
3.1
3.2
3.3
3.4
FOR LOOPS
IF STATEMENTS
WHILE LOOP
INPUT/OUTPUT COMMANDS
SELECTED BIBLIOGRAPHY
EXERCISES
© 1999 CRC Press LLC
CHAPTER FOUR DC ANALYSIS
4.1 NODAL ANALYSIS
4.2 LOOP ANALYSIS
4.3 MAXIMUM POWER TRANSFER
4.3.1 MATLAB diff and find Functions
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER FIVE TRANSIENT ANALYSIS
5.1
5.2
5.3
5.4
RC NETWORK
RL NETWORK
RLC CIRCUIT
STATE VARIABLE APPROACH
5.4.1 MATLAB ode functions
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER SIX
AC ANALYSIS AND NETWORK
FUNCTIONS
6.1 STEADY STATE AC POWER
6.1.1 MATLAB functions quad and quad8
6.2 SINGLE- AND THREE-PHASE AC CIRCUITS
6.3 NETWORK CHARACTERISTICS
6.3.1 MATLAB functions roots, residue and
polyval
6.4 FREQUENCY RESPONSE
6.4.1 MATLAB Function freqs
SELECTED BIBLIOGRAPHY
EXERCISES
© 1999 CRC Press LLC
CHAPTER SEVEN TWO-PORT NETWORKS
7.1 TWO-PORT NETWORK REPRESENTATIONS
7.1.1 z-parameters
7.1.2 y-parameters
7.1.3 h-parameters
7.1.4 Transmission parameters
7.2 INTERCONNECTION OF TWO-PORT
NETWORKS
7.3 TERMINATED TWO-PORT NETWORKS
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER EIGHT FOURIER ANALYSIS
8.1 FOURIER SERIES
8.2 FOURIER TRANSFORMS
8.2.1 Properties of Fourier transform
8.3 DISCRETE AND FAST FOURIER TRANSFORMS
8.3.1 MATLAB function fft
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER NINE
DIODES
9.1 DIODE CHARACTERISTICS
9.1.1 Forward-biased region
9.1.2 MATLAB function polyfit
9.1.3 Temperature effects
9.2 ANALYSIS OF DIODE CIRCUITS
9.3 HALF-WAVE RECTIFIER
9.3.1 MATLAB function fzero
9.4 FULL-WAVE RECTIFICATION
9.5 ZENER DIODE VOLTAGE REGULATOR
CIRCUIT
SELECTED BIBLIOGRAPHY
EXERCISES
© 1999 CRC Press LLC
CHAPTER TEN
SEMICONDUCTOR PHYSICS
10.1 INTRINSIC SEMICONDUCTOR
10.1.1 Energy bands
10.1.2 Mobile carriers
10.2 EXTRINSIC SEMICONDUCTOR
10.2.1 Electron and hole concentrations
10.2.2 Fermi level
10.2.3 Current density and mobility
10.3 PN JUNCTION: CONTACT POTENTIAL,
JUNCTION CURRENT
10.3.1 Contact potential
10.3.2 Junction current
10.4 DEPLETION AND DIFFUSION
CAPACITANCES
10.4.1 Depletion capacitance
10.4.2 Diffusion capacitance
10.5 BREAKDOWN VOLTAGES OF PN JUNCTIONS
REFERENCES
EXERCISES
CHAPTER ELEVEN OPERATIONAL AMPLIFIERS
11.1
11.2
11.3
11.4
11.5
11.6
PROPERTIES OF THE OP AMP
INVERTING CONFIGURATION
NON-INVERTING CONFIGURATION
EFFECT OF FINITE OPEN-LOOP GAIN
FREQUENCY RESPONSE OF OP AMPS
SLEW RATE AND FULL-POWER
BANDWIDTH
11.7 COMMON-MODE REJECTION
SELECTED BIBLIOGRAPHY
EXERCISES
CHAPTER TWELVE TRANSISTOR CIRCUITS
12.1 BIPOLAR JUNCTION TRANSISTORS
12.2 BIASING OF BJT DISCRETE CIRCUITS
12.2.1 Self-bias circuit
12.2.2 Bias stability
12.3 INTEGRATED CIRCUIT BIASING
12.3.1 Simple current mirror
© 1999 CRC Press LLC
12.3.2 Wilson current source
12.4 FREQUENCY RESPONSE OF
COMMON EMITTER AMPLIFIER
12.5 MOSFET CHARACTERISTICS
12.6 BIASING OF MOSFET CIRCUITS
12.7 FREQUENCY RESPONSE OF
COMMON-SOURCE AMPLIFIER
SELECTED BIBLIOGRAPHY
EXERCISES
© 1999 CRC Press LLC
LIST OF EXAMPLES IN TEXT
CHAPTER ONE
EXAMPLE
MATLAB FUNDAMENTALS
DESCRIPTION
1.1
Power Dissipation in a Resistor
1.2
Complex Number Representation
1.3
Equivalent Resistance
1.4
Quadratic Equation
CHAPTER TWO PLOTTING COMMANDS
EXAMPLE
DESCRIPTION
2.1
2.2
Voltage and Current of an RL Circuit
Gain versus Frequency of an RC Amplifier
2.3
Polar Plot of a Complex Number
CHAPTER THREE CONTROL STATEMENTS
EXAMPLE
© 1999 CRC Press LLC
DESCRIPTION
3.1
Horizontal and Vertical Displacement
3.2
A 3-bit A/D Converter
3.3
Consecutive Integer Numbers
CHAPTER FOUR DC ANALYSIS
EXAMPLE
DESCRIPTION
4.1
Nodal Voltages of a Simple Circuit
4.2
4.3
Circuit with Dependent and Independent
Sources
Loop Analysis of a Bridge Circuit
4.4
Power Dissipation and Source Current
4.5
4.6
Nodal Voltage Circuit with Dependent Sources
Maximum Power Dissipation
CHAPTER FIVE TRANSIENT ANALYSIS
EXAMPLE
5.1
5.2
5.3
Charging of a Capacitor with Different Time
Constants
Charging and Discharging of a Capacitor
5.4
Current Flowing through Inductor of RL
Circuit
Current Flowing through a Series RLC Circuit
5.5
Voltage across a Parallel RLC Circuit
5.6
State Variable Approach to RC Circuit
5.7
State Variable Approach to an RLC Circuit
Analysis
State Variable Analysis of a Network
5.8
© 1999 CRC Press LLC
DESCRIPTION
CHAPTER SIX
EXAMPLE
AC ANALYSIS AND NETWORK FUNCTIONS
DESCRIPTION
6.1
Power Calculations of One-port Network
6.2
6.3
6.4
AC Voltage of an RLC Circuit
AC Current and Voltage of a Circuit with Two
Sources
Unbalanced Wye-wye Connection
6.5
Network Function, Poles and Zeros of a Circuit
6.6
Inverse Laplace Transform
6.7
Magnitude and Phase Response of an RLC
Circuit
CHAPTER SEVEN TWO-PORT NETWORKS
EXAMPLE
7.1
z-parameters of T-Network
7.2
y-parameters of Pi-Network
7.3
y-parameters of Field Effect Transistor
7.4
h-parameters of Bipolar Junction Transistor
7.5
Transmission Parameters of a Simple
Impedance Network
Transmission Parameters of a Simple
Admittance Network
y-parameters of Bridge T-Network
7.6
7.7
7.8
7.9
7.10
© 1999 CRC Press LLC
DESCRIPTION
Transmission Parameters of a Simple
Cascaded Network
Transmission Parameters of a Cascaded System
z - parameters and Magnitude Responses of an
Active Lowpass Filter
CHAPTER EIGHT FOURIER ANALYSIS
EXAMPLE
DESCRIPTION
8.1
Fourier Series Expansion of a Square Wave
8.2
8.3
Amplitude and Phase Spectrum of Full-wave
Rectifier Waveform
Synthesis of a Periodic Exponential Signal
8.4
DFT and FFT of a Sequence
8.5
Fourier Transform and DFT of a Damped
Exponential Sinusoid
Power Spectral Density of a Noisy Signal
8.6
CHAPTER NINE
EXAMPLE
DESCRIPTION
9.1
Determination of Diode Parameters from Data
9.2
I-V characteristic of a Diode at Different
Temperatures
Operating Point of a Diode Using Graphical
Techniques
Operating Point of a Diode using Iterative
Technique
Battery Charging Circuit – Current, Conduction
Angle and Peak Current
Capacitor Smoothing Circuit – Calculation of
Critical Times
Full-wave Rectifier – Ripple Voltage, Dc
Output Voltage, Discharge Time and Period of
Ripple
A Zener Diode Voltage Regulator
9.3
9.4
9.5
9.6
9.7
9.8
© 1999 CRC Press LLC
DIODES
CHAPTER TEN
EXAMPLE
SEMICONDUCTOR PHYSICS
DESCRIPTION
10.1
10.2
Electron Concentration versus Temperature
Minority Carriers in Doped Semiconductor
10.3
10.4
Electron and Hole Mobilities versus Doping
Concentration
Resistivity versus Doping
10.5
Junction Potential versus Voltage
10.6
Effects of Temperature on Reverse Saturation
Current
Depletion Capacitance of a PN Junction
10.7
10.8
Diffusion and Depletion Capacitance as a
Function of Voltage
10.9
Effect of Doping Concentration on the
Breakdown Voltage of a PN Junction
CHAPTER ELEVEN OPERATIONAL AMPLIFIERS
EXAMPLE
© 1999 CRC Press LLC
DESCRIPTION
11.1
Frequency Response of Miller Integrator
11.2
11.3
Transfer function, Poles, and Zeros of a Noninverting Op Amp Circuit
Effect of Finite Open Loop Gain
11.4
Open Loop Gain Characteristics of an Op Amp
11.5
11.6
Effect of Closed Loop Gain on the Frequency
Response of an Op Amp
Output Voltage versus Full-power Bandwidth
11.7
Effect of CMRR on the Closed Loop Gain
CHAPTER TWELVE TRANSISTOR CIRCUITS
EXAMPLE
12.1
12.2
Input Characteristics of a BJT
Output Characteristics of an NPN Transistor
12.3
Self-Bias Circuit – Stability Factors and
Collector Current as a Function of
Temperature
Comparison of Simple Current Mirror and
Wilson Current Source
Frequency Response of a Common Emitter
Amplifier
I-V Characteristics of NMOS
12.4
12.5
12.6
12.7
12.8
12.9
© 1999 CRC Press LLC
DESCRIPTION
Operating Point Calculation of NMOS Biasing
Circuit
Voltage and Current Calculations for a
MOSFET Current mirror
Common-source Amplifier Gain, Cut-off
Frequencies and Bandwidth
CHAPTER ONE
MATLAB FUNDAMENTALS
MATLAB is a numeric computation software for engineering and scientific
calculations. The name MATLAB stands for MATRIX LABORATORY.
MATLAB is primarily a tool for matrix computations. It was developed by
John Little and Cleve Moler of MathWorks, Inc. MATLAB was originally
written to provide easy access to the matrix computation software packages
LINPACK and EISPACK.
MATLAB is a high-level language whose basic data type is a matrix that does
not require dimensioning. There is no compilation and linking as is done in
high-level languages, such as C or FORTRAN. Computer solutions in
MATLAB seem to be much quicker than those of a high-level language such
as C or FORTRAN. All computations are performed in complex-valued double precision arithmetic to guarantee high accuracy.
MATLAB has a rich set of plotting capabilities. The graphics are integrated in
MATLAB. Since MATLAB is also a programming environment, a user can
extend the functional capabilities of MATLAB by writing new modules.
MATLAB has a large collection of toolboxes in a variety of domains. Some
examples of MATLAB toolboxes are control system, signal processing, neural
network, image processing, and system identification. The toolboxes consist
of functions that can be used to perform computations in a specific domain.
1.1
MATLAB BASIC OPERATIONS
When MATLAB is invoked, the command window will display the prompt >>.
MATLAB is then ready for entering data or executing commands. To quit
MATLAB, type the command
exit or quit
MATLAB has on-line help. To see the list of MATLAB’s help facility, type
help
The help command followed by a function name is used to obtain information on a specific MATLAB function. For example, to obtain information on
the use of fast Fourier transform function, fft, one can type the command
© 1999 CRC Press LLC
help fft
The basic data object in MATLAB is a rectangular numerical matrix with real
or complex elements. Scalars are thought of as a 1-by-1 matrix. Vectors are
considered as matrices with a row or column. MATLAB has no dimension
statement or type declarations. Storage of data and variables is allocated
automatically once the data and variables are used.
MATLAB statements are normally of the form:
variable = expression
Expressions typed by the user are interpreted and immediately evaluated by the
MATLAB system. If a MATLAB statement ends with a semicolon, MATLAB
evaluates the statement but suppresses the display of the results. MATLAB
is also capable of executing a number of commands that are stored in a file.
This will be discussed in Section 1.6. A matrix
1 2 3
A= 2 3 4
3 4 5
may be entered as follows:
A = [1 2 3; 2 3 4; 3 4 5];
Note that the matrix entries must be surrounded by brackets [
] with row
elements separated by blanks or by commas. The end of each row, with the
exception of the last row, is indicated by a semicolon. A matrix A can also be
entered across three input lines as
A=[123
234
3 4 5];
In this case, the carriage returns replace the semicolons.
four elements
B = [ 6 9 12 15 18 ]
can be entered in MATLAB as
© 1999 CRC Press LLC
A row vector B with
B = [6 9 12 15 18];
or
B = [6 , 9,12,15,18]
For readability, it is better to use spaces rather than commas between the elements. The row vector B can be turned into a column vector by transposition,
which is obtained by typing
C = B’
The above results in
C=
6
9
12
15
18
Other ways of entering the column vector C are
C = [6
9
12
15
18]
or
C = [6; 9; 12; 15; 18]
MATLAB is case sensitive in naming variables, commands and functions.
Thus b and B are not the same variable. If you do not want MATLAB to be
case sensitive, you can use the command
casesen off
To obtain the size of a specific variable, type size ( ). For example, to find the
size of matrix A, you can execute the following command:
size(A)
© 1999 CRC Press LLC
The result will be a row vector with two entries. The first is the number of
rows in A, the second the number of columns in A.
To find the list of variables that have been used in a MATLAB session, type
the command
whos
There will be a display of variable names and dimensions. Table 1.1 shows
the display of the variables that have been used so far in this book:
Table 1.1
Display of an output of whos command
Name
Size
Elements
Byte
Density
Complex
A
B
C
ans
3 by 3
1 by 5
5 by 1
1 by 2
9
5
5
2
72
40
40
16
Full
Full
Full
Full
No
No
No
No
The grand total is 21 elements using 168 bytes.
Table 1.2 shows additional MATLAB commands to get one started on
MATLAB. Detailed descriptions and usages of the commands can be obtained
from the MATLAB help facility or from MATLAB manuals.
Table 1.2
Some Basic MATLAB Commands
Command
%
demo
length
clear
clc
clg
diary
© 1999 CRC Press LLC
Description
Comments. Everything appearing after % command is not executed.
Access on-line demo programs
Length of a matrix
Clears the variables or functions from workspace
Clears the command window during a work session
Clears graphic window
Saves a session in a disk, possibly for printing at a
later date
1.2
MATRIX OPERATIONS
The basic matrix operations are addition(+), subtraction(-), multiplication (*),
and conjugate transpose(‘) of matrices. In addition to the above basic operations, MATLAB has two forms of matrix division: the left inverse operator \
or the right inverse operator /.
Matrices of the same dimension may be subtracted or added. Thus if E and F
are entered in MATLAB as
E = [7 2 3; 4 3 6; 8 1 5];
F = [1 4 2; 6 7 5; 1 9 1];
and
G=E-F
H=E+F
then, matrices G and H will appear on the screen as
G=
6 -2
-2 -4
7 -8
1
1
4
H=
8 6 5
10 10 11
9 10 6
A scalar (1-by-1 matrix) may be added to or subtracted from a matrix. In this
particular case, the scalar is added to or subtracted from all the elements of another matrix. For example,
J=H+1
gives
J=
9
11
10
7 6
11 12
11 7
Matrix multiplication is defined provided the inner dimensions of the two operands are the same. Thus, if X is an n-by-m matrix and Y is i-by-j matrix,
© 1999 CRC Press LLC
X*Y is defined provided m is equal to i. Since E and F are 3-by-3 matrices,
the product
Q = E*F
results as
Q=
22
28
19
69
91
84
27
29
26
Any matrix can be multiplied by a scalar. For example,
2*Q
gives
ans =
44 138
56 182
38 168
54
58
52
Note that if a variable name and the “=” sign are omitted, a variable name ans
is automatically created.
Matrix division can either be the left division operator \ or the right division
operator /. The right division a/b, for instance, is algebraically equivalent to
a
b
while the left division a\b is algebraically equivalent to .
b
a
If Z * I = V and Z is non-singular, the left division, Z\V is equivalent to
MATLAB expression
I = inv ( Z ) * V
where inv is the MATLAB function for obtaining the inverse of a matrix. The
right division denoted by V/Z is equivalent to the MATLAB expression
I = V * inv ( Z )
There are MATLAB functions that can be used to produce special matrices.
Examples are given in Table 1.3.
© 1999 CRC Press LLC
Table 1.3
Some Utility Matrices
Function
ones(n,m)
Description
Produces n-by-m matrix with all the elements being
unity
gives n-by-n identity matrix
Produces n-by-m matrix of zeros
Produce a vector consisting of diagonal of a square
matrix A
eye(n)
zeros(n,m)
diag(A)
1.3
ARRAY OPERATIONS
Array operations refer to element-by-element arithmetic operations. Preceding
the linear algebraic matrix operations, * / \ ‘ , by a period (.) indicates an array
or element-by-element operation. Thus, the operators .* , .\ , ./, .^ , represent
element-by-element multiplication, left division, right division, and raising to
the power, respectively. For addition and subtraction, the array and matrix operations are the same. Thus, + and .+ can be regarded as an array or matrix
addition.
If A1 and B1 are matrices of the same dimensions, then A1.*B1 denotes an array whose elements are products of the corresponding elements of A1 and B1.
Thus, if
A1 = [2 7 6
8 9 10];
B1 = [6 4 3
2 3 4];
then
C1 = A1.*B1
results in
C1 =
12
16
© 1999 CRC Press LLC
28
27
18
40
An array operation for left and right division also involves element-by-element
operation. The expressions A1./B1 and A1.\B1 give the quotient of elementby-element division of matrices A1 and B1. The statement
D1 = A1./B1
gives the result
D1 =
0.3333
4.0000
1.7500
3.0000
2.0000
2.5000
0.5714
0.3333
0.5000
0.4000
and the statement
E1 = A1.\B1
gives
E1 =
3.0000
0.2500
The array operation of raising to the power is denoted by .^. The general
statement will be of the form:
q = r1.^s1
If r1 and s1 are matrices of the same dimensions, then the result q is also a matrix of the same dimensions. For example, if
r1 = [ 7 3 5];
s1 = [ 2 4 3];
then
q1 = r1.^s1
gives the result
q1 =
49
© 1999 CRC Press LLC
81 125
One of the operands can be scalar. For example,
q2 = r1.^2
q3 = (2).^s1
will give
q2 =
49
9
25
and
q3 =
4
16
8
Note that when one of the operands is scalar, the resulting matrix will have the
same dimensions as the matrix operand.
1.4
COMPLEX NUMBERS
MATLAB allows operations involving complex numbers. Complex numbers
are entered using function i or j. For example, a number z = 2 + j 2 may be
entered in MATLAB as
z = 2+2*i
or
z = 2+2*j
Also, a complex number za
za = 2 2 exp[(π / 4) j ]
can be entered in MATLAB as
za = 2*sqrt(2)*exp((pi/4)*j)
It should be noted that when complex numbers are entered as matrix elements
within brackets, one should avoid any blank spaces.
For example,
y = 3 + j 4 is represented in MATLAB as
© 1999 CRC Press LLC
y = 3+4*j
If spaces exist around the + sign, such as
u= 3 + 4*j
MATLAB considers it as two separate numbers, and y will not be equal to u.
If w is a complex matrix given as
1 + j1 2 − j 2
3 + j 2 4 + j3
w=
then we can represent it in MATLAB as
w = [1+j 2-2*j; 3+2*j 4+3*j]
which will produce the result
w=
1.0000 + 1.0000i 2.0000 - 2.0000i
3.0000 + 2.0000i 4.0000 + 3.0000i
If the entries in a matrix are complex, then the “prime” (‘) operator produces
the conjugate transpose. Thus,
wp = w'
will produce
wp =
1.0000 - 1.0000i 3.0000 - 2.0000i
2.0000 + 2.0000i 4.0000 - 3.0000i
For the unconjugate transpose of a complex matrix, we can use the point transpose (.’) command. For example,
wt = w.'
will yield
© 1999 CRC Press LLC
wt =
1.0000 + 1.0000i 3.0000 + 2.0000i
2.0000 - 2.0000i 4.0000 + 3.0000i
1.5
THE COLON SYMBOL (:)
The colon symbol (:) is one of the most important operators in MATLAB. It
can be used (1) to create vectors and matrices, (2) to specify sub-matrices and
vectors, and (3) to perform iterations. The statement
t1 = 1:6
will generate a row vector containing the numbers from 1 to 6 with unit increment. MATLAB produces the result
t1 =
1
2
3
4
5
6
Non-unity, positive or negative increments, may be specified. For example,
the statement
t2 = 3:-0.5:1
will result in
t2 =
3.0000
2.5000
2.0000
1.5000
1.0000
6.0000
4.4000
8.0000 10.0000
4.2000 4.0000
The statement
t3 = [(0:2:10);(5:-0.2:4)]
will result in a 2-by-4 matrix
t3 =
0
5.0000
2.0000
4.8000
4.0000
4.6000
Other MATLAB functions for generating vectors are linspace and logspace.
Linspace generates linearly evenly spaced vectors, while logspace generates
© 1999 CRC Press LLC
logarithmically evenly spaced vectors. The usage of these functions is of the
form:
linspace(i_value, f_value, np)
logspace(i_value, f_value, np)
where
i_value is the initial value
f_value is the final value
np is the total number of elements in the vector.
For example,
t4 = linspace(2, 6, 8)
will generate the vector
t4 =
Columns 1 through 7
2.0000
5.4286
2.5714
3.1429
3.7143
4.2857
4.8571
Column 8
6.0000
Individual elements in a matrix can be referenced with subscripts inside parentheses. For example, t2(4) is the fourth element of vector t2. Also, for matrix
t3, t3(2,3) denotes the entry in the second row and third column. Using the colon as one of the subscripts denotes all of the corresponding row or column.
For example, t3(:,4) is the fourth column of matrix t3. Thus, the statement
t5 = t3(:,4)
will give
t5 =
6.0000
4.4000
© 1999 CRC Press LLC
Also, the statement t3(2,:) is the second row of matrix t3. That is the statement
t6 = t3(2,:)
will result in
t6 =
5.0000
4.8000
4.6000
4.4000
4.2000
4.0000
If the colon exists as the only subscript, such as t3(:), the latter denotes the
elements of matrix t3 strung out in a long column vector. Thus, the statement
t7 = t3(:)
will result in
t7 =
0
5.0000
2.0000
4.8000
4.0000
4.6000
6.0000
4.4000
8.0000
4.2000
10.0000
4.0000
Example 1.1
The voltage, v, across a resistance is given as (Ohm’s Law), v = Ri , where
i is the current and R the resistance. The power dissipated in resistor R is
given by the expression
P = Ri 2
© 1999 CRC Press LLC
If R = 10 Ohms and the current is increased from 0 to 10 A with increments
of 2A, write a MATLAB program to generate a table of current, voltage and
power dissipation.
Solution:
MATLAB Script
diary ex1_1.dat
% diary causes output to be written into file ex1_1.dat
% Voltage and power calculation
R=10;
% Resistance value
i=(0:2:10); % Generate current values
v=i.*R;
% array multiplication to obtain voltage
p=(i.^2)*R; % power calculation
sol=[i v p] % current, voltage and power values are printed
diary
% the last diary command turns off the diary state
MATLAB produces the following result:
sol =
Columns 1 through 6
0
2
4
6
8
10
Columns 7 through 12
0
20
40
60
80
360
640
100
Columns 13 through 18
0
40
160
1000
Columns 1 through 6 constitute the current values, columns 7 through 12 are
the voltages, and columns 13 through 18 are the power dissipation values.
© 1999 CRC Press LLC
1.6
M-FILES
Normally, when single line commands are entered, MATLAB processes the
commands immediately and displays the results. MATLAB is also capable of
processing a sequence of commands that are stored in files with extension m.
MATLAB files with extension m are called m-files. The latter are ASCII text
files, and they are created with a text editor or word processor. To list m-files
in the current directory on your disk, you can use the MATLAB command
what. The MATLAB command, type, can be used to show the contents of a
specified file. M-files can either be script files or function files. Both script
and function files contain a sequence of commands. However, function files
take arguments and return values.
1.6.1
Script files
Script files are especially useful for analysis and design problems that require
long sequences of MATLAB commands. With script file written using a text
editor or word processor, the file can be invoked by entering the name of the
m-file, without the extension. Statements in a script file operate globally on
the workspace data. Normally, when m-files are executing, the commands are
not displayed on screen. The MATLAB echo command can be used to view
m-files while they are executing. To illustrate the use of script file, a script
file will be written to simplify the following complex valued expression z.
Example 1.2
Simplify the complex number z and express it both in rectangular and polar
form.
z=
(3 + j 4)(5 + j 2)(2∠60 0 )
(3 + j 6)(1 + j 2)
Solution:
The following program shows the script file that was used to evaluate the
complex number, z, and express the result in polar notation and rectangular
form.
MATLAB Script
diary ex1_2.dat
© 1999 CRC Press LLC
% Evaluation of Z
% the complex numbers are entered
Z1 = 3+4*j;
Z2 = 5+2*j;
theta = (60/180)*pi; % angle in radians
Z3 = 2*exp(j*theta);
Z4 = 3+6*j;
Z5 = 1+2*j;
% Z_rect is complex number Z in rectangular form
disp('Z in rectangular form is'); % displays text inside brackets
Z_rect = Z1*Z2*Z3/(Z4+Z5);
Z_rect
Z_mag = abs (Z_rect); % magnitude of Z
Z_angle = angle(Z_rect)*(180/pi); % Angle in degrees
disp('complex number Z in polar form, mag, phase'); % displays text
%inside brackets
Z_polar = [Z_mag, Z_angle]
diary
The program is named ex1_2.m. It is included in the disk that accompanies
this book. Execute it by typing ex1_2 in the MATLAB command window.
Observe the result, which should be
Z in rectangular form is
Z_rect =
1.9108 + 5.7095i
complex number Z in polar form (magnitude and phase) is
Z_polar =
6.0208 71.4966
1.6.2
Function Files
Function files are m-files that are used to create new MATLAB functions.
Variables defined and manipulated inside a function file are local to the function, and they do not operate globally on the workspace. However, arguments
may be passed into and out of a function file.
The general form of a function file is
© 1999 CRC Press LLC
function variable(s) = function_name (arguments)
% help text in the usage of the function
%
.
.
end
To illustrate the usage of function files and rules for writing m-file function, let
us study the following two examples.
Example 1.3
Write a function file to solve the equivalent resistance of series connected resistors, R1, R2, R3, …, Rn.
Solution:
MATLAB Script
function req = equiv_sr(r)
% equiv_sr is a function program for obtaining
%
the equivalent resistance of series
%
connected resistors
% usage: req = equiv_sr(r)
%
r is an input vector of length n
%
req is an output, the equivalent resistance(scalar)
%
n = length(r); % number of resistors
req = sum (r); % sum up all resistors
end
The above MATLAB script can be found in the function file equiv_sr.m,
which is available on the disk that accompanies this book.
Suppose we want to find the equivalent resistance of the series connected resistors 10, 20, 15, 16 and 5 ohms. The following statements can be typed in the
MATLAB command window to reference the function equiv_sr
a = [10 20 15 16 5];
Rseries = equiv_sr(a)
diary
The result obtained from MATLAB is
© 1999 CRC Press LLC
Rseries =
66
Example 1.4
Write a MATLAB function to obtain the roots of the quadratic equation
ax 2 + bx + c = 0
Solution:
MATLAB Script
function rt = rt_quad(coef)
%
% rt_quad is a function for obtaining the roots of
%
of a quadratic equation
% usage: rt = rt_quad(coef)
%
coef is the coefficients a,b,c of the quadratic
%
equation ax*x + bx + c =0
%
rt are the roots, vector of length 2
% coefficient a, b, c are obtained from vector coef
a = coef(1); b = coef(2); c = coef(3);
int = b^2 - 4*a*c;
if int > 0
srint = sqrt(int);
x1= (-b + srint)/(2*a);
x2= (-b - srint)/(2*a);
elseif int == 0
x1= -b/(2*a);
x2= x1;
elseif int < 0
srint = sqrt(-int);
p1 = -b/(2*a);
p2 = srint/(2*a);
x1 = p1+p2*j;
x2 = p1-p2*j;
end
rt =[x1;
x2];
end
© 1999 CRC Press LLC
The above MATLAB script can be found in the function file rt_quad.m, which
is available on the disk that accompanies this book.
We can use m-file function, rt_quad, to find the roots of the following quadratic equations:
(a) x2 + 3x + 2 = 0
(b) x2 + 2x + 1 = 0
(c) x2 -2x +3 = 0
The following statements, that can be found in the m-file ex1_4.m, can be
used to obtain the roots:
diary ex1_4.dat
ca = [1 3 2];
ra = rt_quad(ca)
cb = [1 2 1];
rb = rt_quad(cb)
cc = [1 -2 3];
rc = rt_quad(cc)
diary
Type into the MATLAB command window the statement ex1_4 and observe
the results. The following results will be obtained:
ra =
-1
-2
rb =
-1
-1
rc=
1.0000 + 1.4142i
1.0000 - 1.4142i
The following is a summary of the rules for writing MATLAB m-file functions:
(1)
The word, function, appears as the first word in a function file. This
is followed by an output argument, an equal sign and the function name. The
© 1999 CRC Press LLC
arguments to the function follow the function name and are enclosed within parentheses.
(2)
The information that follows the function, beginning with the % sign,
shows how the function is used and what arguments are passed. This information is displayed if help is requested for the function name.
(3)
MATLAB can accept multiple input arguments and multiple output
arguments can be returned.
(4)
If a function is going to return more than one value, all the values
should be returned as a vector in the function statement. For example,
function [mean, variance] = data_in(x)
will return the mean and variance of a vector x. The mean and variance are
computed with the function.
(5)
If a function has multiple input arguments, the function statement
must list the input arguments. For example,
function [mean, variance] = data(x,n)
will return mean and variance of a vector x of length n.
(6)
The last statement in the function file should be an “end” statement.
SELECTED BIBLIOGRAPHY
1.
MathWorks, Inc., MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Biran, A. and Breiner, M., MATLAB for Engineers, AddisonWesley, 1995.
3.
Etter, D.M., Engineering Problem Solving with MATLAB, 2nd Edition, Prentice Hall, 1997.
© 1999 CRC Press LLC
EXERCISES
1.1
The voltage across a discharging capacitor is
v ( t ) = 10(1 − e −0.2 t )
Generate a table of voltage, v ( t ) , versus time, t, for t = 0 to 50
seconds with increment of 5 s.
1.2
Use MATLAB to evaluate the complex number
z=
(3 + j 6)( 6 + j 4)
+ 7 + j10
( 2 + j1) j 2
1.3
Write a function-file to obtain the dot product and the vector product
of two vectors a and b. Use the function to evaluate the dot and
vector products of vectors x and y, where x = (1 5 6) and
y = (2 3 8).
1.4
Write a function-file that can be used to calculate the equivalent
resistance of n parallel connected resistors. In general, the equivalent
resistance of resistors R1 , R2 , R3 , ...., Rn is given by
1
1
1
1
1
=
+
+
+ ... +
Req R1 R2 R3
Rn
1.5
The voltage V is given as V = RI , where R and I are resistance
matrix and I current vector. Evaluate V given that
1 2 4
R = 2 3 6
3 6 7
1.6
and
1
I = 2
6
Use MATLAB to simplify the expression
. e j 0.6 + (3 + j 6 )e j 0.3π
y = 0.5 + j 6 + 35
© 1999 CRC Press LLC
1.7
Write a function file to evaluate n factorial (i.e. n!); where
n ! = n( n − 1)( n − 2)..( 2)(1)
7!
Use the function to compute x =
3! 4 !
1.8
For a triangle with sides of length a, b, and c, the area A is given as
A = s( s − a )( s − b )( s − c )
where
s = (a + b + c ) / 2
Write a function to compute the area given the sides of a triangle.
Use the function to compute the area of triangles with the lengths:
(a) 56, 27 and 43 (b) 5, 12 and 13.
© 1999 CRC Press LLC
CHAPTER TWO
PLOTTING COMMANDS
2.1
GRAPH FUNCTIONS
MATLAB has built-in functions that allow one to generate bar charts, x-y,
polar, contour and 3-D plots, and bar charts. MATLAB also allows one to
give titles to graphs, label the x- and y-axes, and add a grid to graphs. In
addition, there are commands for controlling the screen and scaling. Table 2.1
shows a list of MATLAB built-in graph functions. One can use MATLAB’s
help facility to get more information on the graph functions.
Table 2.1
Plotting Functions
© 1999 CRC Press LLC
FUNCTION
DESRIPTION
axis
bar
contour
ginput
grid
gtext
histogram
hold
loglog
mesh
meshdom
pause
plot
polar
semilogx
semilogy
shg
stairs
text
title
xlabel
ylabel
freezes the axis limits
plots bar chart
performs contour plots
puts cross-hair input from mouse
adds grid to a plot
does mouse positioned text
gives histogram bar graph
holds plot (for overlaying other plots)
does log versus log plot
performs 3-D mesh plot
domain for 3-D mesh plot
wait between plots
performs linear x-y plot
performs polar plot
does semilog x-y plot (x-axis logarithmic)
does semilog x-y plot (y-axis logarithmic)
shows graph screen
performs stair-step graph
positions text at a specified location on graph
used to put title on graph
labels x-axis
labels y-axis
2.2
X-Y PLOTS AND ANNOTATIONS
The plot command generates a linear x-y plot. There are three variations of the
plot command.
(a)
plot(x)
(b)
plot(x, y)
(c)
plot(x1, y1, x2, y2, x3, y3, ..., xn, yn)
If x is a vector, the command
plot(x)
will produce a linear plot of the elements in the vector x as a function of the
index of the elements in x. MATLAB will connect the points by straight lines.
If x is a matrix, each column will be plotted as a separate curve on the same
graph. For example, if
x = [ 0 3.7 6.1 6.4 5.8 3.9 ];
then, plot(x) results in the graph shown in Figure 2.1.
If x and y are vectors of the same length, then the command
plot(x, y)
plots the elements of x (x-axis) versus the elements of y (y-axis). For example,
the MATLAB commands
t = 0:0.5:4;
y = 6*exp(-2*t);
plot(t,y)
will plot the function y ( t ) = 6e
The plot is shown in Figure 2.2.
−2 t
at the following times: 0, 0.5, 1.0, …, 4 .
To plot multiple curves on a single graph, one can use the plot command
with multiple arguments, such as
plot(x1, y1, x2, y2, x3, y3, ..., xn, yn)
© 1999 CRC Press LLC
Figure 2.1 Graph of a Row Vector x
The variables x1, y1, x2, y2, etc., are pairs of vector. Each x-y pair is
graphed, generating multiple lines on the plot. The above plot command
allows vectors of different lengths to be displayed on the same graph.
MATLAB automatically scales the plots. Also, the plot remains as the current
plot until another plot is generated; in which case, the old plot is erased. The
hold command holds the current plot on the screen, and inhibits erasure and
rescaling. Subsequent plot commands will overplot on the original curves.
The hold command remains in effect until the command is issued again.
When a graph is drawn, one can add a grid, a title, a label and x- and y-axes
to the graph. The commands for grid, title, x-axis label, and y-axis label are
grid (grid lines), title (graph title), xlabel (x-axis label), and ylabel (y-axis
label), respectively. For example, Figure 2.2 can be titled, and axes labeled
with the following commands:
t = 0:0.5:4;
y = 6*exp(-2*t);
plot(t, y)
title('Response of an RC circuit')
xlabel('time in seconds')
ylabel('voltage in volts')
grid
© 1999 CRC Press LLC
Figure 2.3 shows the graph of Figure 2.2 with title, x-axis, y-axis and grid
added.
Figure 2.2 Graph of Two Vectors t and y
To write text on a graphic screen beginning at a point (x, y) on the graphic
screen, one can use the command
text(x, y, ’text’)
For example, the statement
text(2.0, 1.5, ’transient analysis’)
will write the text, transient analysis, beginning at point (2.0,1.5). Multiple
text commands can be used. For example, the statements
plot(a1,b1,a2,b2)
text(x1,y1,’voltage’)
text(x2,y2,’power’)
© 1999 CRC Press LLC
will provide texts for two curves: a1 versus b1 and a2 versus b2. The text will
be at different locations on the screen provided x1 ≠ x2 or y1 ≠ y2.
If the default line-types used for graphing are not satisfactory, various symbols
may be selected. For example:
plot(a1, b1, ’*’)
draws a curve, a1 versus b1, using star(*) symbols, while
plot(a1, b1, ’*’, a2, b2, ’+’)
uses a star(*) for the first curve and the plus(+) symbol for the second curve.
Other print types are shown in Table 2.2.
Figure 2.3 Graph of Voltage versus Time of a Response of an RLC
Circuit
For systems that support color, the color of the graph may be specified using
the statement:
plot(x, y, ’g’)
© 1999 CRC Press LLC
implying, plot x versus y using green color. Line and mark style may be added
to color type using the command
plot(x, y, ’+w’)
The above statement implies plot x versus y using white + marks. Other colors
that can be used are shown in Table 2.3.
Table 2.2
Print Types
LINE-TYPES
INDICATORS
solid
dash
dotted
dashdot
-:
-.
POINT
TYPES
point
plus
star
circle
x-mark
INDICATORS
.
+
*
o
x
Table 2.3
Symbols for Color Used in Plotting
COLOR
red
green
blue
white
invisible
SYMBOL
r
g
b
w
i
The argument of the plot command can be complex. If z is a complex vector,
then plot(z) is equivalent to plot(real(z), imag(z)). The following example
shows the use of the plot, title, xlabel, ylabel and text functions.
Example 2.1
For an R-L circuit, the voltage
v (t ) and current i (t ) are given as
v ( t ) = 10 cos(377t )
i( t ) = 5 cos(377t + 60 0 )
© 1999 CRC Press LLC
Sketch
v (t ) and i (t ) for t = 0 to 20 milliseconds.
Solution
MATLAB Script
% RL circuit
% current i(t) and voltage v(t) are generated; t is time
t = 0:1E-3:20E-3; v = 10*cos(377*t);
a_rad = (60*pi/180); % angle in radians
i = 5*cos(377*t + a_rad);
plot(t,v,'*',t,i,'o')
title('Voltage and Current of an RL circuit')
xlabel('Sec')
ylabel('Voltage(V) and Current(mA)')
text(0.003, 1.5, 'v(t)');
text(0.009,2, 'i(t)')
Figure 2.4 shows the resulting graph. The file ex2_1.m is a script file for the
solution of the problem.
Figure 2.4 Plot of Voltage and Current of an RL Circuit under
Sinusoidal Steady State Conditions
© 1999 CRC Press LLC
2.3
LOGARITHMIC AND POLAR PLOTS
Logarithmic and semi-logarithmic plots can be generated using the commands
loglog, semilogx, and semilogy. The use of the above plot commands is
similar to those of the plot command discussed in the previous section. The
description of these commands are as follows:
loglog(x, y) - generates a plot of log10(x) versus log10(y)
semilogx(x, y) - generates a plot of log10(x) versus linear axis of y
semilogy(x, y) - generates a plot of linear axis of x versus log10(y)
It should be noted that since the logarithm of negative numbers and zero does
not exist, the data to be plotted on the semi-log axes or log-log axes should not
contain zero or negative values.
Example 2.2
The gain versus frequency of a capacitively coupled amplifier is shown below.
Draw a graph of gain versus frequency using a logarithmic scale for the
frequency and a linear scale for the gain.
Frequency
(Hz)
20
40
80
100
120
Gain (dB)
5
10
30
32
34
Frequency
(Hz)
2000
5000
8000
10000
12000
Gain (dB)
34
34
34
32
30
Solution
MATLAB Script
% Bode plot for capacitively coupled amplifier
f = [20 40 80 100 120 2000 5000 8000 10000 ...
12000 15000 20000];
g = [ 5 10 30 32 34 34 34 34 32 30 10 5];
semilogx(f, g)
© 1999 CRC Press LLC
title('Bode plot of an amplifier')
xlabel('Frequency in Hz')
ylabel('Gain in dB')
The plot is shown in Figure 2.5. The MATLAB script file is ex2_2.m.
Figure 2.5 Plot of Gain versus Frequency of an Amplifier
A polar plot of an angle versus magnitude may be generated using the
command
polar(theta, rho)
where,
theta and rho are vectors, with the theta being an angle in radians and
rho being the magnitude.
© 1999 CRC Press LLC
When the grid command is issued after the polar plot command, polar grid
lines will be drawn. The polar plot command is used in the following example.
Example 2.3
z = re jθ . The n th power of
0
n
n jnθ
the complex number is given as z = r e . If r = 1.2 and θ = 10 , use
n
the polar plot to plot z versus nθ for n = 1 to n = 36.
A complex number z can be represented as
Solution
MATLAB Script
% polar plot of z
r = 1.2; theta = 10*pi/180;
angle = 0:theta:36*theta; mag = r.^(angle/theta);
polar(angle,mag)
grid
title('Polar Plot')
The polar plot is shown in Figure 2.6.
Figure 2.6 Polar Plot of
© 1999 CRC Press LLC
. n e j10 n
z = 12
2.4
SCREEN CONTROL
MATLAB has basically two display windows: a command window and a graph
window. The hardware configuration an operator is using will either display
both windows simultaneously or one at a time. The following commands can
be used to select and clear the windows:
shg
any key
clc
clg
home
-
shows graph window
brings back command window
clears command window
clears graph window
home command cursor
The graph window can be partitioned into multiple windows. The subplot
command allows one to split the graph window into two subdivisions or four
subdivisions. Two sub-windows can be arranged either top or bottom or left or
right. A four-window partition will have two sub-windows on top and two subwindows on the bottom. The general form of the subplot command is
subplot(i j k)
The digits i and j specify that the graph window is to be split into an i-by- j
th
grid of smaller windows. The digit k specifies the k window for the current
plot. The sub-windows are numbered from left to right, top to bottom. For
example,
%
x = -4:0.5:4;
y = x.^2; % square of x
z = x.^3; % cube of x
subplot(211), plot(x, y), title('square of x')
subplot(212), plot(x, z), title('cube of x')
will plot y = x
in the top half of the graph screen and z = x will be
plotted on the bottom half of the graph screen. The plots are shown in Figure
2.7.
2
© 1999 CRC Press LLC
3
Figure 2.7 Plots of
x 2 and x 3 using Subplot Commands.
The coordinates of points on the graph window can be obtained using the
ginput command. There are two forms of the command:
[x y] = ginput
[x y] = ginput(n)
© 1999 CRC Press LLC
•
[x y] = ginput command allows one to select an unlimited number of
points from the graph window using a mouse or arrow keys. Pressing the
return key terminates the input.
•
[x y] = ginput(n) command allows the selection of n points from the graph
window using a mouse or arrow keys. The points are stored in vectors x
and y. Data points are entered by pressing a mouse button or any key on
the keyboard (except return key). Pressing the return key terminates the
input.
SELECTED BIBLIOGRAPHY
1.
MathWorks, Inc, MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Biran, A. and Breiner, M. MATLAB for Engineers, AddisonWesley, 1995.
3.
Etter, D.M., Engineering Problem Solving with MATLAB,
Edition, Prentice Hall, 1997.
2nd
EXERCISES
2.1
The repulsive coulomb force that exists between two protons in the
nucleus of a conductor is given as
F=
q1q2
4πε 0 r 2
1
= 8.99 x109 Nm2 / C 2 ,
4πε0
sketch a graph of force versus radius r. Assume a radius from
. x10 −14 m with increments of 2.0 x10 −15 m.
10
. x10 −15 to 10
If
2.2
. x10 −19 C, and
q1 = q 2 = 16
The current flowing through a drain of a field effect transistor during
saturation is given as
iDS = k (VGS − Vt ) 2
k = 2.5mA / V 2 , plot the current i DS
for the following values of VGS : 1.5, 2.0, 2.5, ..., 5 V.
If
2.3
. volt and
Vt = 10
Plot the voltage across a parallel RLC circuit given as
v ( t ) = 5e 2 t sin(1000πt )
© 1999 CRC Press LLC
z = r − n e jnθ for θ = 150 and n = 1 to
2.4
Obtain the polar plot of
20.
2.5
The table below shows the grades of three examinations of ten
students in a class.
STUDENT
1
2
3
4
5
6
7
8
9
10
EXAM #1
81
75
95
65
72
79
93
69
83
87
EXAM #2
78
77
90
69
73
84
97
72
80
81
EXAM #3
83
80
93
72
71
86
94
67
82
77
(a) Plot the results of each examination.
(b) Use MATLAB to calculate the mean and standard deviation of
each examination.
2.6
A function
f ( x ) is given as
f ( x ) = x 4 + 3x 3 + 4 x 2 + 2 x + 6
(a) Plot
f ( x ) and
(b) Find the roots of
2.7
f ( x)
A message signal m(t) and the carrier signal
communication system are, respectively:
c(t ) of a
m( t ) = 4 cos(120πt ) + 2 cos( 240πt )
c( t ) = 10 cos(10,000πt )
A double-sideband suppressed carrier
© 1999 CRC Press LLC
s(t ) is given as
s( t ) = m( t )c( t )
Plot
2.8
m(t ), c(t ) and s(t ) using the subplot command.
The voltage v and current I of a certain diode are related by the
expression
i = I S exp[v / ( nVT )]
−14
. x10 A, n = 2.0 and VT = 26 mV, plot the current
If I S = 10
versus voltage curve of the diode for diode voltage between 0 and 0.6
volts.
© 1999 CRC Press LLC
CHAPTER THREE
CONTROL STATEMENTS
3.1
FOR LOOPS
“FOR” loops allow a statement or group of statements to be repeated a fixed
number of times. The general form of a for loop is
for index = expression
statement group X
end
The expression is a matrix and the statement group X is repeated as many
times as the number of elements in the columns of the expression matrix. The
index takes on the elemental values in the matrix expression. Usually, the expression is something like
m:n or m:i:n
where m is the beginning value, n the ending value, and i is the increment.
Suppose we would like to find the squares of all the integers starting from 1 to
100. We could use the following statements to solve the problem:
sum = 0;
for i = 1:100
sum = sum + i^2;
end
sum
For loops can be nested, and it is recommended that the loop be indented for
readability. Suppose we want to fill 10-by-20 matrix, b, with an element value
equal to unity, the following statements can be used to perform the operation.
%
n = 10;
% number of rows
m = 20;
% number of columns
for i = 1:n
for j = 1:m
b(i,j) = 1; % semicolon suppresses printing in the loop
end
end
© 1999 CRC Press LLC
b
%
% display the result
It is important to note that each for statement group must end with the word
end. The following program illustrates the use of a for loop.
Example 3.1
The horizontal displacement
with respect to time, t, as
x (t ) and vertical displacement y (t ) are given
x ( t ) = 2t
y( t ) = sin( t )
For t = 0 to 10 ms, determine the values of
plot
x (t ) and y (t ) . Use the values to
x (t ) versus y (t ) .
Solution:
MATLAB Script
%
for i= 0:10
x(i+1) = 2*i;
y(i+1) = 2*sin(i);
end
plot(x,y)
Figure 3.1 shows the plots of
© 1999 CRC Press LLC
x (t ) and y (t ) .
Figure 3.1 Plot of x versus y.
3.2
IF STATEMENTS
IF statements use relational or logical operations to determine what steps to
perform in the solution of a problem. The relational operators in MATLAB
for comparing two matrices of equal size are shown in Table 3.1.
Table 3.1
Relational Operators
RELATIONAL
OPERATOR
<
<=
>
>=
==
~=
© 1999 CRC Press LLC
MEANING
less than
less than or equal
greater than
greater than or equal
equal
not equal
When any of the above relational operators are used, a comparison is done between the pairs of corresponding elements. The result is a matrix of ones and
zeros, with one representing TRUE and zero FALSE. For example, if
a = [1 2 3 3 3 6];
b = [1 2 3 4 5 6];
a == b
The answer obtained is
ans =
1
1
1
0
0
1
The 1s indicate the elements in vectors a and b that are the same and 0s are the
ones that are different.
There are three logical operators in MATLAB. These are shown in Table 3.2.
Table 3.2
Logical Operators
LOGICAL OPERATOR
SYMBOL
&
!
~
MEANING
and
or
not
Logical operators work element-wise and are usually used on 0-1 matrices,
such as those generated by relational operators. The & and ! operators compare two matrices of equal dimensions. If A and B are 0-1 matrices, then A&B
is another 0-1 matrix with ones representing TRUE and zeros FALSE. The
NOT(~) operator is a unary operator. The expression ~C returns 1 where C is
zero and 0 when C is nonzero.
There are several variations of the IF statement:
© 1999 CRC Press LLC
•
simple if statement
•
nested if statement
•
if-else statement
•
•
if-elseif statement
•
if-elseif-else statement.
The general form of the simple if statement is
if logical expression 1
statement group 1
end
In the case of a simple if statement, if the logical expression1 is true, the statement group 1 is executed. However, if the logical expression is false, the
statement group 1 is bypassed and the program control jumps to the statement
that follows the end statement.
•
The general form of a nested if statement is
if logical expression 1
statement group 1
if logical expression 2
statement group 2
end
statement group 3
end
statement group 4
The program control is such that if expression 1 is true, then statement groups
1 and 3 are executed. If the logical expression 2 is also true, the statement
groups 1 and 2 will be executed before executing statement group 3. If logical
expression 1 is false, we jump to statement group 4 without executing statement groups 1, 2 and 3.
•
The if-else statement allows one to execute one set of statements if a
logical expression is true and a different set of statements if the logical
statement is false. The general form of the if-else statement is
if logical expression 1
statement group 1
else
statement group 2
end
© 1999 CRC Press LLC
In the above program segment, statement group 1 is executed if logical expression 1 is true. However, if logical expression 1 is false, statement group 2 is
executed.
•
If-elseif statement may be used to test various conditions before executing a set of statements. The general form of the if-elseif statement is
if logical expression 1
statement group1
elseif logical expression 2
statement group2
elseif logical expression 3
statement group 3
elseif logical expression 4
statement group 4
end
A statement group is executed provided the logical expression above it is true.
For example, if logical expression 1 is true, then statement group 1 is executed.
If logical expression 1 is false and logical expression 2 is true, then statement
group 2 will be executed. If logical expressions 1, 2 and 3 are false and logical
expression 4 is true, then statement group 4 will be executed. If none of the
logical expressions is true, then statement groups 1, 2, 3 and 4 will not be executed. Only three elseif statements are used in the above example. More elseif
statements may be used if the application requires them.
•
If-elseif-else statement provides a group of statements to be executed if
other logical expressions are false. The general form of the if-elseif-else
statement is
if logical expression 1
statement group1
elseif logical expression 2
statement group 2
elseif logical expression 3
statement group 3
elseif logical expression 4
statement group4
else
statement group 5
end
© 1999 CRC Press LLC
The various logical expressions are tested. The one that is satisfied is executed. If the logical expressions 1, 2, 3 and 4 are false, then statement group 5
is executed. Example 3.2 shows the use of the if-elseif-else statement.
Example 3.2
A 3-bit A/D converter, with an analog input x and digital output y, is represented by the equation:
y=0
=1
=2
=3
=4
=5
=6
=7
x < -2.5
-2.5 ≤ x < -1.5
-1.5 ≤ x < -0.5
-0.5 ≤ x < 0.5
0.5 ≤ x < 1.5
1.5 ≤ x < 2.5
2.5 ≤ x < 3.5
x ≥ 3.5
Write a MATLAB program to convert analog signal x to digital signal y. Test
the program by using an analog signal with the following amplitudes: -1.25,
2.57 and 6.0.
Solution
MATLAB Script
diary ex3_2.dat
%
y1 = bitatd_3(-1.25)
y2 = bitatd_3(2.57)
y3 = bitatd_3(6.0)
diary
function Y_dig = bitatd_3(X_analog)
%
% bitatd_3 is a function program for obtaining
%
the digital value given an input analog
%
signal
%
% usage: Y_dig = bitatd_3(X_analog)
%
Y_dig is the digital number (in integer form)
© 1999 CRC Press LLC
%
X_analog is the analog input (in decimal form)
%
if X_analog < -2.5
Y_dig = 0;
elseif X_analog >= -2.5 & X_analog < -1.5
Y_dig = 1;
elseif X_analog >= -1.5 & X_analog < -0.5
Y_dig = 2;
elseif X_analog >= -0.5 & X_analog < 0.5
Y_dig = 3;
elseif X_analog >= 0.5 & X_analog < 1.5
Y_dig = 4;
elseif X_analog >= 1.5 & X_analog < 2.5
Y_dig = 5;
elseif X_analog >= 2.5 & X_analog < 3.5
Y_dig = 6;
else
Y_dig = 7;
end
Y_dig;
end
The function file, bitatd_3.m, is an m-file available in the disk that accompanies this book. In addition, the script file, ex3_2.m on the disk, can be used to
perform this example. The results obtained, when the latter program is executed, are
y1 =
2
y2 =
6
y3 =
7
3.3
WHILE LOOP
A WHILE loop allows one to repeat a group of statements as long as a specified condition is satisfied. The general form of the WHILE loop is
© 1999 CRC Press LLC
while expression 1
statement group 1
end
statement group 2
When expression 1 is true, statement group 1 is executed. At the end of executing the statement group 1, the expression 1 is retested. If expression 1 is
still true, the statement group 1 is again executed. However, if expression 1 is
false, the program exits the while loop and executes statement group 2. The
following example illustrates the use of the while loop.
Example 3.3
Determine the number of consecutive integer numbers which when added together will give a value equal to or just less than 210.
Solution
MATLAB Script
diary ex3_3.dat
% integer summation
int = 1; int_sum = 0;
max_val = 210;
while int_sum < max_val
int_sum = int_sum + int;
int = int + 1;
end
last_int = int
if int_sum == max_val
num_int = int - 1
tt_int_ct = int_sum
elseif int_sum > max_val
num_int = int - 1
tt_int_ct = int_sum - last_int
end
end
diary
The solution obtained will be
last_int =
21
© 1999 CRC Press LLC
num_int =
20
tt_int_ct =
210
Thus, the number of integers starting from 1 that would add up to 210 is 20.
That is,
1 + 2 + 3 + 4 + ... + 20 = 210
3.4
INPUT/OUTPUT COMMANDS
MATLAB has commands for inputting information in the command window
and outputting data. Examples of input/output commands are echo, input,
pause, keyboard, break, error, display, format, and fprintf. Brief descriptions
of these commands are shown in Table 3.3.
Table 3.3
Some Input/output Commands
COMMAND
DESCRIPTION
break
disp
echo
error
format
exits while or for loops
displays text or matrix
displays m-files during execution
displays error messages
switches output display to a particular
format
displays text and matrices and specifies
format for printing values
allows user input
invokes the keyboard as an m-file
causes an m-file to stop executing. Pressing any key cause resumption of program
execution.
fprintf
input
keyboard
pause
Break
The break command may be used to terminate the execution of for and while
loops. If the break command exits in an innermost part of a nested loop, the
© 1999 CRC Press LLC
break command will exit from that loop only. The break command is useful in
exiting a loop when an error condition is detected.
Disp
The disp command displays a matrix without printing its name. It can also be
used to display a text string. The general form of the disp command is
disp(x)
disp(‘text string’)
disp(x) will display the matrix x. Another way of displaying matrix x is to type
its name. This is not always desirable since the display will start with a leading
“x = ”. Disp(‘text string’) will display the text string in quotes. For example, the MATLAB statement
disp(‘3-by-3 identity matrix’)
will result in
3-by-3 identity matrix
and
disp(eye(3,3))
will result in
1
0
0
0
1
0
0
0
1
Echo
The echo command can be used for debugging purposes. The echo command
allows commands to be viewed as they execute. The echo can be enabled or
disabled.
echo on echo off echo -
© 1999 CRC Press LLC
enables the echoing of commands
disables the echoing of commands
by itself toggles the echo state
Error
The error command causes an error return from the m-files to the keyboard
and displays a user written message. The general form of the command is
error(‘message for display’)
Consider the following MATLAB statements:
x = input(‘Enter age of student’);
if x < 0
error(‘wrong age was entered, try again’)
end
x = input(‘Enter age of student’)
For the above MATLAB statements, if the age is less that zero, the error message ‘wrong age was entered, try again’ will be displayed and the user will
again be prompted for the correct age.
Format
The format controls the format of an output. Table 3.4 shows some formats
available in MATLAB.
Table 3.4
Format Displays
COMMAND
MEANING
format short
format long
format short e
format long e
format hex
format +
5 significant decimal digits
15 significant digits
scientific notation with 5 significant digits
scientific notation with 15 significant digits
hexadecimal
+ printed if value is positive, - if negative; space is
skipped if value is zero
By default, MATLAB displays numbers in “short” format (5 significant digits). Format compact suppresses line-feeds that appear between matrix displays, thus allowing more lines of information to be seen on the screen. For-
© 1999 CRC Press LLC
mat loose reverts to the less compact display. Format compact and format
loose do not affect the numeric format.
fprintf
The fprintf can be used to print both text and matrix values. The format for
printing the matrix can be specified, and line feed can also be specified. The
general form of this command is
fprintf(‘text with format specification’, matrices)
For example, the following statements
cap = 1.0e-06;
fprintf('The value of capacitance is %7.3e Farads\n', cap)
when executed will yield the output
The value of capacitance is 1.000e-006 Farads
The format specifier %7.3e is used to show where the matrix value should be
printed in the text. 7.3e indicates that the capacitance value should be printed
with an exponential notation of 7 digits, three of which should be decimal
digits. Other format specifiers are
%f %g -
floating point
signed decimal number in either %e or %f format,
whichever is shorter
The text with format specification should end with \n to indicate the end of
line. However, we can also use \n to get line feeds as represented by the following example:
r1 = 1500;
fprintf('resistance is \n%f Ohms \n', r1)
the output is
resistance is
1500.000000 Ohms
© 1999 CRC Press LLC
Input
The input command displays a user-written text string on the screen, waits for
an input from the keyboard, and assigns the number entered on the keyboard as
the value of a variable. For example, if one types the command
r = input(‘Please enter the four resistor values’);
when the above command is executed, the text string ‘Please, enter the four
resistor values’ will be displayed on the terminal screen. The user can then
type an expression such as
[10 15 30 25];
The variable r will be assigned a vector [10 15 30 25]. If the user strikes the
return key, without entering an input, an empty matrix will be assigned to r.
To return a string typed by a user as a text variable, the input command may
take the form
x = input(‘Enter string for prompt’, ’s’)
For example, the command
x = input(‘What is the title of your graph’, ’s’)
when executed, will echo on the screen, ‘What is the title of your graph.’ The
user can enter a string such as ‘Voltage (mV) versus Current (mA).’
Keyboard
The keyboard command invokes the keyboard as an m-file. When the word
keyboard is placed in an m-file, execution of the m-file stops when the word
keyboard is encountered. MATLAB commands can then be entered. The
keyboard mode is terminated by typing the word, “return” and pressing the
return key. The keyboard command may be used to examine or change a variable or may be used as a tool for debugging m-files.
© 1999 CRC Press LLC
Pause
The pause command stops the execution of m-files. The execution of the mfile resumes upon pressing any key. The general forms of the pause command
are
pause
pause(n)
Pause stops the execution of m-files until a key is pressed. Pause(n) stops the
execution of m-files for n seconds before continuing. The pause command can
be used to stop m-files temporarily when plotting commands are encountered
during program execution. If pause is not used, the graphics are momentarily
visible.
SELECTED BIBLIOGRAPHY
1.
MathWorks, Inc., MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Biran, A. and Breiner, M., MATLAB for Engineers, AddisonWesley, 1995.
3.
Etter, D.M., Engineering Problem Solving with MATLAB, 2nd
Edition, Prentice Hall, 1997.
EXERCISES
3.1
Write a MATLAB program to add all the even numbers from 0 to
100.
3.2
Add all the terms in the series
1+
1 1 1
+ + + ...
2 4 8
until the sum exceeds 1.995. Print out the sum and the number of
terms needed to just exceed the sum of 1.995.
© 1999 CRC Press LLC
3.3
The Fibonacci sequence is given as
1 1 2 3 5 8 13 21 34 …
Write a MATLAB program to generate the Fibonacci sequence up
to the twelfth term. Print out the results.
3.4
The table below shows the final course grade and its corresponding
relevant letter grade.
LETTER GRADE
A
B
C
D
F
FINAL COURSE GRADE
90 < grade ≤ 100
80 < grade ≤ 90
70 < grade ≤ 80
60 < grade ≤ 70
grade ≤ 60
For the course grades: 70, 85, 90, 97, 50, 60, 71, 83, 91, 86, 77, 45,
67, 88, 64, 79, 75, 92, and 69
(a) Determine the number of students who attained the grade of A
and F.
(b) What are the mean grade and the standard deviation?
3.5
Write a script file to evaluate y[1], y[2], y[3] and y[4] for the
difference equation:
y[n] = 2 y[n − 1] − y[n − 2] + x[n]
n ≥ 0. Assume that x[n] = 1 for n ≥ 0, y[ −2] = 2 and
y[ −1] = 1.
for
3.6
The equivalent impedance of a circuit is given as
Z eq ( jw) = 100 + jwL +
1
jwC
If L = 4 H and C = 1 µF,
(a) Plot
Z eq ( jw) versus w. (b) What is the minimum impedance?
(c) With what frequency does the minimum impedance occur?
© 1999 CRC Press LLC
CHAPTER FOUR
DC ANALYSIS
4.1
NODAL ANALYSIS
Kirchhoff’s current law states that for any electrical circuit, the algebraic sum
of all the currents at any node in the circuit equals zero. In nodal analysis, if
there are n nodes in a circuit, and we select a reference node, the other nodes
can be numbered from V1 through Vn-1. With one node selected as the reference node, there will be n-1 independent equations. If we assume that the admittance between nodes i and j is given as Yij , we can write the nodal equations:
Y11 V1 + Y12 V2 + …
+ Y1m Vm =
∑
I1
Y21 V1 + Y22 V2 + …
+ Y2m Vm =
∑
I2
Ym1 V1 + Ym2 V2 + …
+ Ymm Vm =
∑
Im
(4.1)
where
m=n-1
V1, V2 and Vm are voltages from nodes 1, 2 and so on ..., n with respect to the reference node.
∑
Ix is the algebraic sum of current sources at node x.
Equation (4.1) can be expressed in matrix form as
[Y ][V ] = [ I ]
(4.2)
The solution of the above equation is
[V ] = [Y ] −1 [ I ]
where
© 1999 CRC Press LLC
(4.3)
[Y ] −1
[Y ] .
is an inverse of
In MATLAB, we can compute [V] by using the command
V = inv (Y ) * I
(4.4)
where
inv (Y ) is the inverse of matrix Y
The matrix left and right divisions can also be used to obtain the nodal voltages. The following MATLAB commands can be used to find the matrix [V]
V = IY
(4.5)
V =Y\I
(4.6)
or
The solutions obtained from Equations (4.4) to (4.6) will be the same, provided the system is not ill-conditioned. The following two examples illustrate
the use of MATLAB for solving nodal voltages of electrical circuits.
Example 4.1
For the circuit shown below, find the nodal voltages V1 ,
V2 and V 3 .
20 Ohms
V
5A
10 Ohms
V
2
40 Ohms
1
50 Ohms
Figure 4.1 Circuit with Nodal Voltages
© 1999 CRC Press LLC
V
3
2A
Solution
Using KCL and assuming that the currents leaving a node are positive, we
have
For node 1,
V1 − V2 V1 − V3
+
−5= 0
10
20
i.e.,
015
. V1 − 01
. V2 − 0.05V3 = 5
(4.7)
At node 2,
V2 − V1 V2 V2 − V3
+
+
=0
10
50
40
i.e.,
. V1 + 0145
. V2 − 0.025V3 = 0
−01
(4.8)
At node 3,
V3 − V1 V3 − V2
+
−2=0
20
40
i.e.,
−0.05V1 − 0.025V2 + 0.075V3 = 2
(4.9)
In matrix form, we have
.
.
−01
−0.05 V1
015
−01
.
0145
.
−0.025 V2 =
−0.05 −0.025 0.075 V3
5
0
2
The MATLAB program for solving the nodal voltages is
MATLAB Script
diary ex4_1.dat
% program computes the nodal voltages
© 1999 CRC Press LLC
(4.10)
% given the admittance matrix Y and current vector I
% Y is the admittance matrix and I is the current vector
% initialize matrix y and vector I using YV=I form
Y = [ 0.15 -0.1 -0.05;
-0.1
0.145 -0.025;
-0.05 -0.025 0.075];
I = [5;
0;
2];
% solve for the voltage
fprintf('Nodal voltages V1, V2 and V3 are \n')
v = inv(Y)*I
diary
The results obtained from MATLAB are
Nodal voltages V1, V2 and V3,
v=
404.2857
350.0000
412.8571
Example 4.2:
Find the nodal voltages of the circuit shown below.
2 Ohms
Ix
V
5 Ohms
1
5A
V
10 I x
2
20 Ohms
V
15 Ohms
3
4 Ohms
V4
10 Ohms
Figure 4.2 Circuit with Dependent and Independent Sources
© 1999 CRC Press LLC
10 V
Solution
Using KCL and the convention that currents leaving a node is positive, we
have
At node 1
V1 V1 − V2 V1 − V4
+
+
−5= 0
20
5
2
Simplifying, we get
0.75V1 − 0.2V2 − 0.5V4 = 5
(4.11)
At node 2,
V2 − V3 = 10 I X
But
IX =
(V1 − V4 )
2
Thus
V2 − V3 =
10(V1 − V4 )
2
Simplifying, we get
- 5V1
+ V2 − V3 + 5V4 = 0
(4.12)
From supernodes 2 and 3, we have
V3 V2 − V1 V2 V3 − V4
+
+
+
=0
10
5
4
15
Simplifying, we get
.
V3 − 0.06667V4 = 0
−0.2V1 + 0.45V2 + 01667
© 1999 CRC Press LLC
(4.13)
At node 4, we have
V4 = 10
(4.14)
In matrix form, equations (4.11) to (4.14) become
0
− 0.5 V1
0.75 − 0.2
V
−5
1
5
−1
2 =
− 0.2 0.45 01667
.
− 0.06667 V3
0
0
1
V4
0
5
0
0
10
The MATLAB program for solving the nodal voltages is
MATLAB Script
diary ex4_2.dat
% this program computes the nodal voltages
% given the admittance matrix Y and current vector I
% Y is the admittance matrix
% I is the current vector
% initialize the matrix y and vector I using YV=I
Y = [0.75 -0.2 0 -0.5;
-5
1 -1
5;
-0.2 0.45 0.166666667 -0.0666666667;
0
0 0
1];
% current vector is entered as a transpose of row vector
I = [5 0 0 10]';
% solve for nodal voltage
fprintf('Nodal voltages V1,V2,V3,V4 are \n')
V = inv(Y)*I
diary
We obtain the following results.
Nodal voltages V1,V2,V3,V4 are
© 1999 CRC Press LLC
(4.15)
V=
18.1107
17.9153
-22.6384
10.0000
4.2
LOOP ANALYSIS
Loop analysis is a method for obtaining loop currents. The technique uses Kirchoff voltage law (KVL) to write a set of independent simultaneous equations.
The Kirchoff voltage law states that the algebraic sum of all the voltages
around any closed path in a circuit equals zero.
In loop analysis, we want to obtain current from a set of simultaneous equations. The latter equations are easily set up if the circuit can be drawn in planar fashion. This implies that a set of simultaneous equations can be obtained
if the circuit can be redrawn without crossovers.
For a planar circuit with n-meshes, the KVL can be used to write equations for
each mesh that does not contain a dependent or independent current source.
Using KVL and writing equations for each mesh, the resulting equations will
have the general form:
Z11I1 + Z12 I2 + Z13 I3 +
...
Z1n In =
∑
V1
Z21 I1 + Z22 I2 + Z23 I3 +
...
Z2n In =
∑
V2
Zn1 I1 + Zn2 I2 + Zn3 I3 +
...
Znn In =
∑
Vn
(4.16)
where
I1, I2, ... In are the unknown currents for meshes 1 through n.
Z11, Z22, …, Znn are the impedance for each mesh through which individual current flows.
Zij, j # i denote mutual impedance.
∑
© 1999 CRC Press LLC
Vx is the algebraic sum of the voltage sources in mesh x.
Equation (4.16) can be expressed in matrix form as
[ Z ][ I ] = [V ]
(4.17)
where
Z11
Z
21
Z = Z31
..
Z n1
Z12
Z22
Z32
..
Zn 2
Z13
Z23
Z33
.
Zn3
...
...
...
...
...
Z1n
Z2 n
Z3 n
..
Z nn
I1
I
2
I = I3
.
I n
and
∑ V1
∑ V2
V = ∑ V3
..
V
∑ n
The solution to Equation (4.17) is
[ I ] = [ Z ] −1 [V ]
(4.18)
In MATLAB, we can compute [I] by using the command
I = inv ( Z ) * V
© 1999 CRC Press LLC
(4.19)
where
inv ( Z ) is the inverse of the matrix Z
The matrix left and right divisions can also be used to obtain the loop currents.
Thus, the current I can be obtained by the MATLAB commands
I =V Z
(4.20)
I = Z \V
(4.21)
or
As mentioned earlier, Equations (4.19) to (4.21) will give the same results,
provided the circuit is not ill-conditioned. The following examples illustrate
the use of MATLAB for loop analysis.
Example 4.3
Use the mesh analysis to find the current flowing through the resistor
addition, find the power supplied by the 10-volt voltage source.
10 Ohms
15 Ohms
RB
10 V
RB . In
5 Ohms
I
30 Ohms
Figure 4.3a Bridge Circuit
© 1999 CRC Press LLC
30 Ohms
Solution
Using loop analysis and designating the loop currents as
the following figure.
I1
I 1 , I 2 , I 3 , we obtain
I2
10 Ohms
15 Ohms
5 Ohms
10 V
I3
30 Ohms
Figure 4.3b
Note that
30 Ohms
Bridge Circuit with Loop Currents
I = I 3 − I 2 and power supplied by the source is P = 10 I1
The loop equations are
Loop 1,
10( I 1 − I 2 ) + 30( I 1 − I 3 ) − 10 = 0
40 I 1 − 10 I 2 − 30 I 3 = 10
Loop 2,
10( I 2 − I 1 ) + 15I 2 + 5( I 2 − I 3 ) = 0
− 10 I 1 + 30 I 2 − 5I 3 = 0
Loop 3,
(4.23)
30( I 3 − I 1 ) + 5( I 3 − I 2 ) + 30 I 3 = 0
− 30 I 1 − 5I 2 + 65I 3 = 0
© 1999 CRC Press LLC
(4.22)
(4.24)
In matrix form, Equations (4.22) and (4.23) become
40 −10 −30 I1
−10 30
−5 I 2 =
−30 −5 65 I3
10
0
0
The MATLAB program for solving the loop currents
and the power supplied by the 10-volt source is
(4.25)
I 1 , I 2 , I 3 , the current I
MATLAB Script
diary ex4_3.dat
% this program determines the current
% flowing in a resistor RB and power supplied by source
% it computes the loop currents given the impedance
% matrix Z and voltage vector V
% Z is the impedance matrix
% V is the voltage matrix
% initialize the matrix Z and vector V
Z = [40 -10 -30;
-10 30 -5;
-30 -5 65];
V = [10 0 0]';
% solve for the loop currents
I = inv(Z)*V;
% current through RB is calculated
IRB = I(3) - I(2);
fprintf('the current through R is %8.3f Amps \n',IRB)
% the power supplied by source is calculated
PS = I(1)*10;
fprintf('the power supplied by 10V source is %8.4f watts \n',PS)
diary
MATLAB answers are
the current through R is 0.037 Amps
the power supplied by 10V source is 4.7531 watts
© 1999 CRC Press LLC
Circuits with dependent voltage sources can be analyzed in a manner similar to
that of example 4.3. Example 4.4 illustrates the use of KVL and MATLAB to
solve loop currents.
Example 4.4
Find the power dissipated by the 8 Ohm resistor and the current supplied by
the 10-volt source.
5V
6 ohms
15 Ohms
10 ohms
Is
6 Ohms
10 V
20 Ohms
4 Is
Figure 4.4a Circuit for Example 4.4
Solution
Using loop analysis and denoting the loop currents as
cuit can be redrawn as
6 Ohms
15 Ohms
5V
10 Ohms
I1
I2
10 V
I3
6 Ohms
8 Ohms
20 Ohms
4 Is
Figure 4.4b
© 1999 CRC Press LLC
I 1 , I 2 and I 3 , the cir-
Figure 4.4 with Loop Currents
By inspection,
I S = I1
(4.26)
For loop 1,
− 10 + 6 I 1 + 20( I 1 − I 2 ) = 0
26 I 1 − 20 I 2 = 10
(4.27)
For loop 2,
15 I 2 − 5 + 6( I 2 − I 3 ) + 4 I S + 20( I 2 − I1 ) = 0
Using Equation (4.26), the above expression simplifies to
− 16 I1 + 41I 2 − 63 I = 5
(4.28)
For loop 3,
10 I 3 + 8 I 3 − 4 I S + 6( I 3 − I 2 ) = 0
Using Equation (4.26), the above expression simplifies to
− 4 I 1 − 6 I 2 + 24 I 3 = 0
(4.29)
Equations (4.25) to (4.27) can be expressed in matrix form as
26 − 20 0 I1
− 16 41 − 6 I 2 =
− 4 − 6 24 I 3
10
5
0
The power dissipated by the 8 Ohm resistor is
P = RI32 = 8 I32
The current supplied by the source is
© 1999 CRC Press LLC
I S = I1
(4.30)
A MATLAB program for obtaining the power dissipated by the 8 Ohm resistor
and the current supplied by the source is shown below
MATLAB Script
diary ex4_4.dat
% This program determines the power dissipated by
% 8 ohm resistor and current supplied by the
% 10V source
%
% the program computes the loop currents, given
% the impedance matrix Z and voltage vector V
%
% Z is the impedance matrix
% V is the voltage vector
% initialize the matrix Z and vector V of equation
% ZI=V
Z = [26 -20 0;
-16 40 -6;
-4 -6 24];
V = [10 5 0]';
% solve for loop currents
I = inv(Z)*V;
% the power dissipation in 8 ohm resistor is P
P = 8*I(3)^2;
% print out the results
fprintf('Power dissipated in 8 ohm resistor is %8.2f Watts\n',P)
fprintf('Current in 10V source is %8.2f Amps\n',I(1))
diary
MATLAB results are
Power dissipated in 8 ohm resistor is 0.42 Watts
Current in 10V source is 0.72 Amps
For circuits that contain both current and voltage sources, irrespective of
whether they are dependent sources, both KVL and KVL can be used to obtain
equations that can be solved using MATLAB. Example 4.5 illustrates one
such circuit.
© 1999 CRC Press LLC
Example 4.5
Find the nodal voltages in the circuit, i.e., V1 ,V2 , ..., V5
10 Ia
V
1
V2
8 Ohms
Ia
V4
10 Ohms
V3
5 Ohms
Vb
4 Ohms
V5
5 Vb
2 Ohms
24 V
5A
Figure 4.5 Circuit for Example 4.5
Solution
By inspection,
Vb = V1 − V4
(4.31)
Using Ohm’s Law
Ia =
V4 − V3
5
(4.32)
Using KCL at node 1, and supernode 1-2, we get
V1 V1 − V4
V − V3
+
− 5Vb + 2
=0
2
10
8
Using Equation (4.31), Equation (4.33) simplifies to
© 1999 CRC Press LLC
(4.33)
. V2 − 0125
. V3 + 4.9V4 = 0
− 4.4V1 + 0125
(4.34)
Using KCL at node 4, we have
V4 − V5 V4 − V3 V4 − V1
+
+
= 10
4
5
10
This simplifies to
. V1 − 0.2V3 + 0.55V4 − 0.25V5 = 0
− 01
(4.35)
Using KCL at node 3, we get
V3 − V4 V3 − V2
+
−5= 0
5
8
which simplifies to
. V2 + 0.325V3 − 0.2V4 = 5
− 0125
(4.36)
Using KVL for loop 1, we have
− 10 I a + Vb + 5I a + 8( I a + 5) = 0
(4.37)
Using Equations (4.31) and (4.32), Equation (4.37) becomes
− 10 I a + Vb + 5I a + 8 I a + 40 = 0
i.e.,
3I a + Vb = −40
Using Equation (4.32), the above expression simplifies to
3
V4 − V3
+ V1 − V4 = −40
5
Simplifying the above expression, we get
V1 − 0.6V3 − 0.4V4 = −40
(4.38)
By inspection
VS = 24
© 1999 CRC Press LLC
(4.39)
Using Equations (4.34), (4.35), (4.36), (4.38) and (4.39), we get the matrix
equation
.
.
4.9
0 V1
−0125
−4.4 0125
−01
.
0
0.55 −0.25 V2
−0.2
0
.
0.325 −0.2
0 V3 =
−0125
0
0 V4
−0.6 −0.4
1
0
0
0
0
1 V5
0
0
5
−40
24
(4.40)
The MATLAB program for obtaining the nodal voltages is shown below.
MATLAB Script
diary ex4_5.dat
% Program determines the nodal voltages
%
given an admittance matrix Y and current vector I
% Initialize matrix Y and the current vector I of
%
matrix equation Y V = I
Y = [-4.4 0.125 -0.125 4.9 0;
-0.1 0
-0.2 0.55 -0.25;
0 -0.125 0.325 -0.2 0;
1 0
-0.6 -0.4 0;
0 0
0
0 1];
I = [0 0 5 -40 24]';
% Solve for the nodal voltages
fprintf('Nodal voltages V(1), V(2), .. V(5) are \n')
V = inv(Y)*I; diary
The results obtained from MATLAB are
Nodal voltages V(1), V(2), ... V(5) are
V=
117.4792
299.7708
193.9375
102.7917
24.0000
© 1999 CRC Press LLC
4.3
MAXIMUM POWER TRANSFER
Assume that we have a voltage source
load
VS with resistance RS connected to a
RL . The circuit is shown in Figure 4.6.
Rs
Vs
RL
VL
Figure 4.6 Circuit for Obtaining Maximum Power Dissipation
The voltage across the Load R L is given as
VL =
Vs RL
Rs + RL
The power dissipated by the load RL is given as
PL =
VL2
Vs2 RL
=
RL ( Rs + RL ) 2
(4.41)
R L that dissipates the maximum power is obtained by differentiating PL with respect to RL , and equating the derivative to zero. That is,
The value of
dPL ( Rs + RL ) 2 VS − Vs RL ( 2)( Rs + RL )
=
dRL
( Rs + RL )4
dPL
=0
dRL
2
© 1999 CRC Press LLC
(4.42)
Simplifying the above we get
( Rs + R L ) − 2 R L = 0
i.e.,
R L = RS
(4.43)
Thus, for a resistive network, the maximum power is supplied to a load provided the load resistance is equal to the source resistance. When R L = 0, the
voltage across and power dissipated by
R L are zero. On the other hand, when
R L approaches infinity, the voltage across the load is maximum, but the
power dissipation is zero. MATLAB can be used to observe the voltage across
and power dissipation of the load as functions of load resistance value. Example 4.6 shows the use of MATLAB to plot the voltage and display the
power dissipation of a resistive circuit.
Before presenting an example on the maximum power transfer theorem, let us
discuss the MATLAB functions diff and find.
4.3.1
MATLAB Diff and Find Functions
Numerical differentiation can be obtained using the backward difference expression
f ′( x n ) =
f ( x n ) − f ( x n−1 )
x n − x n−1
(4.44)
or by the forward difference expression
f ′( x n ) =
The derivative of
as
f ( x n +1 ) − f ( x n )
x n +1 − x n
f ( x ) can be obtained by using the MATLAB diff function
f ′( x ) ≅ diff ( f )./ diff ( x ).
If
© 1999 CRC Press LLC
(4.45)
f is a row or column vector
(4.46)
f = [ f (1)
f ( 2 ) ...
f ( n )]
then diff(f) returns a vector of difference between adjacent elements
diff ( f ) = [ f ( 2 ) − f (1)
f (3) − f ( 2) ...
f ( n ) − f ( n − 1)]
(4.47)
The output vector
diff ( f ) will be one element less than the input vector f .
The find function determines the indices of the nonzero elements of a vector
or matrix. The statement
B = find(
f)
(4.48)
will return the indices of the vector f that are nonzero. For example, to obtain the points where a change in sign occurs, the statement
Pt_change = find(product < 0)
(4.49)
will show the indices of the locations in product that are negative.
The diff and find are used in the following example to find the value of resistance at which the maximum power transfer occurs.
Example 4.6
R L varies from 0 to 50KΩ, plot the power dissipated by the
load. Verify that the maximum power dissipation by the load occurs when R L
In Figure 4.7, as
is 10 KΩ.
© 1999 CRC Press LLC
10,000 Ohms
10 V
PL
Figure 4.7
RL
VL
Resistive Circuit for Example 4.6
Solution
MATLAB Script
% maximum power transfer
% vs is the supply voltage
% rs is the supply resistance
% rl is the load resistance
% vl is the voltage across the load
% pl is the power dissipated by the load
vs = 10; rs = 10e3;
rl = 0:1e3:50e3;
k = length(rl); % components in vector rl
% Power dissipation calculation
for i=1:k
pl(i) = ((vs/(rs+rl(i)))^2)*rl(i);
end
% Derivative of power is calculated using backward difference
dp = diff(pl)./diff(rl);
rld = rl(2:length(rl)); % length of rld is 1 less than that of rl
% Determination of critical points of derivative of power
prod = dp(1:length(dp) - 1).*dp(2:length(dp));
crit_pt = rld(find(prod < 0));
max_power = max(pl); % maximum power is calculated
% print out results
© 1999 CRC Press LLC
fprintf('Maximum power occurs at %8.2f Ohms\n',crit_pt)
fprintf('Maximum power dissipation is %8.4f Watts\n', max_power)
% Plot power versus load
plot(rl,pl,'+')
title('Power delivered to load')
xlabel('load resistance in Ohms')
ylabel('power in watts')
The results obtained from MATLAB are
Maximum power occurs at 10000.00 Ohms
Maximum power dissipation is 0.0025 Watts
The plot of the power dissipation obtained from MATLAB is shown in Figure
4.8.
Figure 4.8 Power delivered to load
© 1999 CRC Press LLC
SELECTED BIBLIOGRAPHY
1.
MathWorks, Inc., MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Etter, D.M., Engineering Problem Solving with MATLAB, 2nd
Edition, Prentice Hall, 1997.
3.
Gottling, J.G., Matrix Analysis of Circuits Using MATLAB,
Prentice Hall, 1995.
4.
Johnson, D.E., Johnson, J.R. and Hilburn, J.L., Electric Circuit
Analysis, 3rd Edition, Prentice Hall, 1997.
5.
Dorf, R.C. and Svoboda, J.A., Introduction to Electric Circuits, 3rd
Edition, John Wiley & Sons, 1996.
EXERCISES
4.1
Use loop analysis to write equations for the circuit shown in Figure
P4.1. Determine the current I using MATLAB.
6 Ohms
4 Ohms
6 Ohms
2 Ohms
10 V
I
8 Ohms
Figure P4.1 Circuit for Exercise 4.1
© 1999 CRC Press LLC
15 Ohms
4.2
Use nodal analysis to solve for the nodal voltages for the circuit
shown in Figure P4.2. Solve the equations using MATLAB.
V2
2 Ohms
V1
6 Ohms
5 Ohms
3A
3 Ohms
V3
V4
4A
4 Ohms
6A
8 Ohms
V5
Figure P4.2 Circuit for Exercise 4.2
4.3
Find the power dissipated by the 4Ω resistor and the voltage V1 .
8A
I
2 Ohms
x
6 Ix
Vo
10 v
Vy
4 Ohms
Figure P4.3
© 1999 CRC Press LLC
2 Ohms
Circuit for Exercise 4.3
3 Vy
4 Ohms
4.4
Using both loop and nodal analysis, find the power delivered by a
15V source.
2A
4 Ohms
4V
5 Ohms
a
8 Ohms
Ix
10 I x
15 V
2 Ohms
Va
Figure P4.4 Circuit for Exercise 4.4
4.5
R L varies from 0 to 12 in increments of 2Ω, calculate the power
dissipated by RL . Plot the power dissipation with respect to the
variation in RL . What is the maximum power dissipated by R L ?
What is the value of R L needed for maximum power dissipation?
As
3 Ohms
2 Ohms
3 Ohms
12 Ohms
12 V
RL
6 Ohms
36 V
Figure P4.5 Circuit for Exercise 4.5
© 1999 CRC Press LLC
4.6
Using loop analysis and MATLAB, find the loop currents. What
is the power supplied by the source?
4 Ohms
3 Ohms
I1
I2
4 Ohms
2 Ohms
2 Ohms
I3
I4
6V
2 Ohms
3 Ohms
Figure P4.6
© 1999 CRC Press LLC
6V
2 Ohms
4 Ohms
Circuit for Exercise 4.6
4 Ohms
CHAPTER FIVE
TRANSIENT ANALYSIS
5.1 RC NETWORK
Considering the RC Network shown in Figure 5.1, we can use KCL to write
Equation (5.1).
R
C
Vo(t)
Figure 5.1 Source-free RC Network
C
dv o ( t ) v o ( t )
+
=0
dt
R
(5.1)
i.e.,
dv o ( t ) v o ( t )
+
=0
dt
CR
If Vm is the initial voltage across the capacitor, then the solution to Equation
(5.1) is
v 0 (t ) = Vm e
t
−
CR
(5.2)
where
CR is the time constant
Equation (5.2) represents the voltage across a discharging capacitor. To obtain
the voltage across a charging capacitor, let us consider Figure 5.2.
© 1999 CRC Press LLC
R
Vs
C
Vo(t)
Figure 5.2 Charging of a Capacitor
Using KCL, we get
C
dv o ( t ) v o ( t ) − Vs
+
=0
dt
R
If the capacitor is initially uncharged, that is
to Equation (5.3) is given as
t
−
v0 (t ) = VS 1 − e CR
(5.3)
v 0 (t ) = 0 at t = 0, the solution
(5.4)
Examples 5.1 and 5.2 illustrate the use of MATLAB for solving problems
related to RC Network.
Example 5.1
Assume that for Figure 5.2 C = 10 µF, use MATLAB to plot the voltage
across the capacitor if R is equal to (a) 1.0 kΩ, (b) 10 kΩ and (c) 0.1 kΩ.
Solution
MATLAB Script
% Charging of an RC circuit
%
c = 10e-6;
r1 = 1e3;
© 1999 CRC Press LLC
tau1 = c*r1;
t = 0:0.002:0.05;
v1 = 10*(1-exp(-t/tau1));
r2 = 10e3;
tau2 = c*r2;
v2 = 10*(1-exp(-t/tau2));
r3 = .1e3;
tau3 = c*r3;
v3 = 10*(1-exp(-t/tau3));
plot(t,v1,'+',t,v2,'o', t,v3,'*')
axis([0 0.06 0 12])
title('Charging of a capacitor with three time constants')
xlabel('Time, s')
ylabel('Voltage across capacitor')
text(0.03, 5.0, '+ for R = 1 Kilohms')
text(0.03, 6.0, 'o for R = 10 Kilohms')
text(0.03, 7.0, '* for R = 0.1 Kilohms')
Figure 5.3 shows the charging curves.
Figure 5.3 Charging of Capacitor
© 1999 CRC Press LLC
From Figure 5.3, it can be seen that as the time constant is small, it takes a
short time for the capacitor to charge up.
Example 5.2
For Figure 5.2, the input voltage is a rectangular pulse with an amplitude of 5V
and a width of 0.5s. If C = 10 µF, plot the output voltage, v 0 ( t ) , for
resistance R equal to (a) 1000 Ω, and (b) 10,000 Ω. The plots should start
from zero seconds and end at 1.5 seconds.
Solution
MATLAB Script
% The problem will be solved using a function program rceval
function [v, t] = rceval(r, c)
% rceval is a function program for calculating
%
the output voltage given the values of
%
resistance and capacitance.
% usage [v, t] = rceval(r, c)
%
r is the resistance value(ohms)
%
c is the capacitance value(Farads)
%
v is the output voltage
%
t is the time corresponding to voltage v
tau = r*c;
for i=1:50
t(i) = i/100;
v(i) = 5*(1-exp(-t(i)/tau));
end
vmax = v(50);
for i = 51:100
t(i) = i/100;
v(i) = vmax*exp(-t(i-50)/tau);
end
end
% The problem will be solved using function program
% rceval
% The output is obtained for the various resistances
c = 10.0e-6;
r1 = 2500;
© 1999 CRC Press LLC
[v1,t1] = rceval(r1,c);
r2 = 10000;
[v2,t2] = rceval(r2,c);
% plot the voltages
plot(t1,v1,'*w', t2,v2,'+w')
axis([0 1 0 6])
title('Response of an RC circuit to pulse input')
xlabel('Time, s')
ylabel('Voltage, V')
text(0.55,5.5,'* is for 2500 Ohms')
text(0.55,5.0, '+ is for 10000 Ohms')
Figure 5.4 shows the charging and discharging curves.
Figure 5.4 Charging and Discharging of a Capacitor with Different
Time Constants
© 1999 CRC Press LLC
5.2
RL NETWORK
Consider the RL circuit shown in Figure 5.5.
L
i(t)
Vo(t)
R
Figure 5.5 Source-free RL Circuit
Using the KVL, we get
L
di( t )
+ Ri( t ) = 0
dt
(5.5)
If the initial current flowing through the inductor is
Equation (5.5) is
i (t ) = I m e
I m , then the solution to
t
−
τ
(5.6)
where
τ = LR
(5.7)
Equation (5.6) represents the current response of a source-free RL circuit with
initial current I m , and it represents the natural response of an RL circuit.
Figure 5.6 is an RL circuit with source voltage
© 1999 CRC Press LLC
v ( t ) = VS .
L
i(t)
V(t)
R
VR(t)
Figure 5.6 RL Circuit with a Voltage Source
Using KVL, we get
L
di (t )
+ Ri (t ) = VS
dt
(5.8)
If the initial current flowing through the series circuit is zero, the solution of
Equation (5.8) is
Rt
−
VS
1 − e L
i(t ) =
R
(5.9)
The voltage across the resistor is
v R (t ) = Ri (t )
Rt
−
= VS 1 − e L
(5.10)
The voltage across the inductor is
v L ( t ) = VS − v R ( t )
= VS e
Rt
−
L
(5.11)
The following example illustrates the use of MATLAB for solving RL circuit
problems.
© 1999 CRC Press LLC
Example 5.3
For the sequential circuit shown in Figure 5.7, the current flowing through the
inductor is zero. At t = 0, the switch moved from position a to b, where it
remained for 1 s. After the 1 s delay, the switch moved from position b to
position c, where it remained indefinitely. Sketch the current flowing through
the inductor versus time.
50 Ohms
b
200 H
a
c
40V
150 Ohms
50 Ohms
Figure 5.7 RL Circuit for Example 5.3
Solution
For 0 < t < 1 s, we can use Equation (5.9) to find the current
t
−
i (t ) = 0.4 1 − e τ1
(5.12)
where
τ 1 = L R = 200 100 = 2 s
At t = 1 s
i( t ) = 0.4(1 − e −0.5 )
= I max
(5.13)
For t > 1 s, we can use Equation (5.6) to obtain the current
i( t ) = I max e
© 1999 CRC Press LLC
t − 0.5
−
τ2
(5.14)
where
τ 2 = L R = 200 200 = 1 s
eq 2
The MATLAB program for plotting
i (t ) is shown below.
MATLAB Script
% Solution to Example 5.3
% tau1 is time constant when switch is at b
% tau2 is the time constant when the switch is in position c
%
tau1 = 200/100;
for k=1:20
t(k) = k/20;
i(k) = 0.4*(1-exp(-t(k)/tau1));
end
imax = i(20);
tau2 = 200/200;
for k = 21:120
t(k) = k/20;
i(k) = imax*exp(-t(k-20)/tau2);
end
% plot the current
plot(t,i,'o')
axis([0 6 0 0.18])
title('Current of an RL circuit')
xlabel('Time, s')
ylabel('Current, A')
Figure 5.8 shows the current
© 1999 CRC Press LLC
i (t ) .
Figure 5.8 Current Flowing through Inductor
5.3
RLC CIRCUIT
For the series RLC circuit shown in Figure 5.9, we can use KVL to obtain
the Equation (5.15).
L
i(t)
Vs(t) = Vs
R
Figure 5.9 Series RLC Circuit
© 1999 CRC Press LLC
Vo(t)
t
di( t ) 1
vS (t ) = L
+ ∫ i(τ )dτ + Ri( t )
dt
C −∞
(5.15)
Differentiating the above expression, we get
dv S (t )
di (t ) i (t )
d 2 i (t )
=L
+R
+
2
dt
C
dt
dt
i.e.,
1 dv S (t ) d 2 i (t ) R di (t ) i (t )
=
+
+
L dt
L dt
LC
dt 2
The homogeneous solution can be found by making
d 2 i( t ) R di( t ) i( t )
0=
+
+
dt 2
L dt
LC
(5.16)
v S (t ) = constant, thus
(5.17)
The characteristic equation is
0 = λ 2 + aλ + b
(5.18)
where
a= RL
and
b = 1 LC
The roots of the characteristic equation can be determined. If we assume that
the roots are
λ = α, β
then, the solution to the homogeneous solution is
i h (t ) = A1e α1t + A2 e α 2 t
where
© 1999 CRC Press LLC
(5.19)
A1 and A2 are constants.
If v S (t ) is a constant, then the forced solution will also be a constant and be
given as
i f (t ) = A3
(5.20)
The total solution is given as
i (t ) = A1e α1t + A2 e α 2 t + A3
(5.21)
where
A1 , A2 , and A3 are obtained from initial conditions.
Example 5.4 illustrates the use of MATLAB for finding the roots of
characteristic equations. The MATLAB function roots, described in Section
6.3.1, is used to obtain the roots of characteristic equations.
Example 5.4
For the series RLC circuit shown in Figure 5.9, If L = 10 H, R = 400 Ohms
and C = 100µF,
v S (t ) = 0, i(0) = 4 A and
di( 0)
= 15 A/s, find i (t ) .
dt
Solution
Since
v S (t ) = 0, we use Equation (5.17) to get
0=
d 2 i( t ) 400 di( t )
+
+ 1000i( t )
10 dt
dt 2
The characteristic equation is
0 = λ2 + 40λ + 1000
The MATLAB function roots is used to obtain the roots of the characteristics
equation.
© 1999 CRC Press LLC
MATLAB Script
p = [1 40 1000];
lambda = roots(p)
lambda =
-20.0000 +24.4949i
-20.0000 -24.4949i
Using the roots obtained from MATLAB,
i (t ) is given as
i (t ) = e −20t ( A1 cos(24.4949t ) + A2 sin(24.4949t )
i (0) = e −0 ( A1 + A2 (0))
⇒ A1 = 4
[
]
di (t )
= −20e − 20t A1 cos(24.4949t ) + A2 sin( 24.4949t ) +
dt
e −20t − 24.4949 A1 sin(24.4949t ) + 24.4949 A2 cos(24.4949t )
[
]
di (0)
= 24.4949 A2 − 20 A1 = 15
dt
Since
.
A1 = 4 , A2 = 38784
.
sin(24.4949t )]
i (t ) = e −20t [4 cos(24.4949t ) + 38784
Perhaps the simplest way to obtain voltages and currents in an RLC circuit is to
use Laplace transform. Table 5.1 shows Laplace transform pairs that are
useful for solving RLC circuit problems.
From the RLC circuit, we write differential equations by using network
analysis tools. The differential equations are converted into algebraic
equations using the Laplace transform. The unknown current or voltage is
then solved in the s-domain. By using an inverse Laplace transform, the
solution can be expressed in the time domain. We will illustrate this method
using Example 5.5
© 1999 CRC Press LLC
Table 5.1
Laplace Transform Pairs
f (t)
f(s)
1
1
1
s
s>0
2
t
1
s2
s>0
3
tn
n!
s n +1
s>0
4
e − at
1
s+a
s>a
5
te − at
1
(s + a) 2
6
sin( wt )
w
s + w2
7
cos( wt )
s
s + w2
8
e at sin( wt )
9
e at cos( wt )
10
df
dt
sF ( s) − f (0 + )
∫ f (τ )dτ
F ( s)
s
f (t − t1 )
e − t1s F ( s)
11
t
0
12
© 1999 CRC Press LLC
2
2
w
(s + a) 2 + w 2
s+a
(s + a) 2 + w 2
s>a
s>0
s>0
Example 5.5
The switch in Figure 5.10 has been opened for a long time. If the switch opens
at t = 0, find the voltage v ( t ) . Assume that R = 10 Ω, L = 1/32 H,
C = 50µF and
I S = 2 A.
t=0
+
Is
R
V(t)
C
L
-
Figure 5.10 Circuit for Example 5.5
At t < 0, the voltage across the capacitor is
v C (0) = (2)(10) = 20 V
In addition, the current flowing through the inductor
i L (0) = 0
At t > 0, the switch closes and all the four elements of Figure 5.10 remain in
parallel. Using KCL, we get
t
v(t )
dv ( t ) 1
IS =
+C
+ ∫ v (τ )dτ + iL ( 0)
R
dt
L0
Taking the Laplace transform of the above expression, we get
I S V ( s)
V ( s) i L (0)
=
+ C[ sV ( s) − VC (0)] +
+
sL
s
s
R
Simplifying the above expression, we get
© 1999 CRC Press LLC
4 Ohms
3 Ohms
I1
I2
6 V
4 Ohms
2 Ohms
2 Ohms
2 Ohms
I3
I4
6 V
2 Ohms
3 Ohms
For
4 Ohms
4 Ohms
I S = 2A, C = 50µ F, R = 10Ω, L = 1/32 H, V ( s) becomes
V ( s) =
40000 + 20 s
s + 2000s + 64 *10 4
V ( s) =
40000 + 20 s
A
B
=
+
( s + 1600 )( s + 400) ( s + 1600) ( s + 400 )
A=
B=
2
Lim
V ( s)( s + 1600) = -6.67
Lim
V ( s )( s + 400)
s→ −1600
= 26.67
s→ −400
v (t ) = −6.67e −1600t + 26.67e −400t
The plot of
v ( t ) is shown in Figure 5.13.
5.4
STATE VARIABLE APPROACH
Another method of finding the transient response of an RLC circuit is the state
variable technique.
The later method (i) can be used to analyze and
synthesize control systems, (ii) can be applied to time-varying and nonlinear
systems, (iii) is suitable for digital and computer solution and (iv) can be used
to develop the general system characteristics.
A state of a system is a minimal set of variables chosen such that if their
values are known at the time t, and all inputs are known for times greater
than t 1 , one can calculate the output of the system for times greater than t 1 .
In general, if we designate x as the state variable, u as the input, and y as
the output of a system, we can express the input u and output y as
© 1999 CRC Press LLC
•
x (t ) = Ax (t ) + Bu(t )
(5.22)
y( t ) = Cx ( t ) + Du( t )
(5.23)
where
u1 ( t )
u ( t )
2
u( t ) = .
.
un ( t )
x1 ( t )
x ( t )
2
x( t ) = .
.
x n ( t )
y1 ( t )
y ( t )
2
y( t ) = .
.
y n ( t )
and A, B, C, and D are matrices determined by constants of a system.
For example, consider a single-input and a single-output system described by
the differential equation
d 4 y( t )
d 3 y( t )
d 2 y( t )
dy( t )
3
4
+
+
+8
+ 2 y( t ) =
4
3
2
dt
dt
dt
dt
6u( t )
(5.24)
We define the components of the state vector as
x1 ( t ) = y( t )
© 1999 CRC Press LLC
x2 ( t ) =
dy( t ) dx1 ( t ) •
=
= x1 ( t )
dt
dt
x3 ( t ) =
d 2 y( t ) dx2 ( t ) •
=
= x2 ( t )
dt 2
dt
x4 ( t ) =
d 3 y( t ) dx3 ( t ) •
=
= x3 ( t )
dt 3
dt
x5 ( t ) =
d 4 y( t ) dx4 ( t ) •
=
= x4 ( t )
dt 4
dt
(5.25)
Using Equations (5.24) and (5.25), we get
•
x 4 ( t ) = 6u( t ) − 3x4 ( t ) − 4 x3 ( t ) − 8 x 2 ( t ) − 2 x1 ( t )
(5.26)
From the Equations (5.25) and (5.26), we get
x •( t )
1•
x ( t )
2• =
x3 ( t )
•
x4 ( t )
0 0 x1 ( t ) 0
0 1
0 0 1 0 x ( t ) 0
2 + u( t )
0 0 0 1 x 3 ( t ) 0
−2 −8 −4 −3 x 4 ( t ) 6
(5.27)
•
or
x (t ) = Ax (t ) + Bu(t )
(5.28)
where
x •( t )
1•
•
x ( t )
x = 2• ; A =
x3 ( t )
•
x 4 ( t )
0 0
0 1
0 0 1 0
;B =
0 0 0 1
−2 −8 −4 −3
0
0
0
6
(5.29)
Since
y ( t ) = x1 ( t )
we can express the output
y (t ) in terms of the state x (t ) and input u(t ) as
y( t ) = Cx ( t ) + Du( t )
where
© 1999 CRC Press LLC
(5.30)
[
]
D = [0]
C = 1 0 0 0 and
(5.31)
In RLC circuits, if the voltage across a capacitor and the current flowing in an
inductor are known at some initial time t, then the capacitor voltage and
inductor current will allow the description of system behavior for all
subsequent times. This suggests the following guidelines for the selection of
acceptable state variables for RLC circuits:
1.
Currents associated with inductors are state variables.
2.
Voltages associated with capacitors are state variables.
3.
Currents or voltages associated with resistors do not specify
independent state variables.
4.
When closed loops of capacitors or junctions of inductors exist in a
circuit, the state variables chosen according to rules 1 and 2 are not
independent.
Consider the circuit shown in Figure 5.11.
R1
+
+
Vs
R3
R2
V1
-
y(t)
+
C1
V2
C2
I1
L
-
Figure 5.11 Circuit for State Analysis
Using the above guidelines, we select the state variables to be V1 ,
Using nodal analysis, we have
© 1999 CRC Press LLC
V2 , and i1 .
C1
dv1 ( t ) V1 − Vs V1 − V2
+
+
=0
dt
R1
R2
(5.32)
C2
dv 2 (t ) V2 − V1
+
+ i1 = 0
dt
R2
(5.33)
Using loop analysis
V2 = i1 R3 + L
The output
di1 ( t )
dt
(5.34)
y (t ) is given as
y ( t ) = v1 ( t ) − v 2 ( t )
(5.35)
Simplifying Equations (5.32) to (5.34), we get
V
V
dv1 ( t )
1
1
)V1 + 2 + s
= −(
+
C1 R2 C1 R1
dt
C1 R1 C1 R2
(5.36)
dv 2 ( t )
V
V
i
= 1 − 2 − 1
dt
C2 R2 C2 R2 C2
(5.37)
di1 ( t ) V2 R3
=
−
i1
dt
L L
(5.38)
Expressing the equations in matrix form, we get
1
1
1
V• − ( C R + C R ) C R
1 1
1 2
1 2
•1
1
1
V 2 =
−
•
C2 R2
C2 R2
i1
1
0
L
0
1
V
1
1 C1 R1
−
V + 0 Vs
C2 2
R i1 0
− 3
L
(5.39)
© 1999 CRC Press LLC
and the output is
V1
y = [1 −1 0]V2
i1
(5.40)
MATLAB functions for solving ordinary differential equations are ODE
functions. These are described in the following section.
5.4.1
MATLAB Ode Functions
MATLAB has two functions, ode23 and ode45, for computing numerical
solutions to ordinary differential equations. The ode23 function integrates a
system of ordinary differential equations using second- and third-order RungeKutta formulas; the ode45 function uses fourth- and fifth-order Runge-Kutta
integration equations.
The general forms of the ode functions are
[ t,x ] = ode23 (xprime, tstart, tfinal, xo, tol,trace)
or
[ t,x ] = ode45 (xprime, tstart, tfinal, xo, tol, trace)
where
xprime is the name (in quotation marks) of the MATLAB function
or m-file that contains the differential equations to be integrated. The
•
function will compute the state derivative vector
x ( t ) given the
current time t, and state vector x ( t ) . The function must have 2 input
arguments, scalar t (time) and column vector x (state) and the
.
function returns the output argument
state derivatives
•
x ( t1 ) =
© 1999 CRC Press LLC
dx ( t1 )
dt
xdot , ( x ) , a column vector of
tstart
is the starting time for the integration
tfinal
is the final time for the integration
xo
is a column vector of initial conditions
tol
is optional. It specifies the desired accuracy of the solution.
Let us illustrate the use of MATLAB ode functions with the following two
examples.
Example 5.6
VS = 10V, R = 10,000 Ω, C = 10µF. Find the output voltage
v 0 (t ) , between the interval 0 to 20 ms, assuming v 0 (0) = 0 and by (a)
For Figure 5.2,
using a numerical solution to the differential equation; and (b) analytical
solution.
Solution
From Equation (5.3), we have
C
dv o ( t ) v o ( t ) − Vs
+
=0
dt
R
thus
dv o ( t ) Vs v o ( t )
=
−
= 100 − 10 v 0 ( t )
dt
CR CR
From Equation(5.4), the analytical solution is
t
−
v 0 (t ) = 10 1 − e CR
MATLAB Script
© 1999 CRC Press LLC
% Solution for first order differential equation
% the function diff1(t,y) is created to evaluate
% the differential equation
% Its m-file is diff1.m
%
% Transient analysis of RC circuit using ode
% function and analytical solution
% numerical solution using ode
t0 = 0;
tf = 20e-3;
xo = 0; % initial conditions
[t, vo] = ode23('diff1',t0,tf,xo);
% the analytical solution given by Equation(5.4) is
vo_analy = 10*(1-exp(-10*t));
% plot two solutions
subplot(121)
plot(t,vo,'b')
title('State Variable Approach')
xlabel('Time, s'),ylabel('Capacitor Voltage, V'),grid
subplot(122)
plot(t,vo_analy,'b')
title('Analytical Approach')
xlabel('Time, s'),ylabel('Capacitor Voltage, V'),grid
%
function dy = diff1(t,y)
dy = 100 - 10*y;
end
Figure 5.12 shows the plot obtained using Equation (5.4) and that obtained
from the MATLAB ode23 function. From the two plots, we can see that the
two results are identical.
© 1999 CRC Press LLC
(a)
(b)
Figure 5.12
Output Voltage v 0 ( t ) Obtained from (a) State
Variable Approach and (b) Analytical Method
Example 5.7
For Figure 5.10, if R = 10Ω, L = 1/32 H, C = 50µF, use a numerical solution
of the differential equation to solve v ( t ) . Compare the numerical solution to
the analytical solution obtained from Example 5.5.
Solution
From Example 5.5,
v C (0) = 20V, i L (0) = 0 , and
di L (t )
= v C (t )
dt
dv (t )
v (t )
C C
+ iL + C
− IS = 0
dt
R
L
© 1999 CRC Press LLC
Simplifying, we get
diL ( t ) v C ( t )
=
dt
L
dv C ( t ) I S iL ( t ) v C ( t )
=
−
−
dt
C
C
RC
Assuming that
x1 (t ) = i L (t )
x 2 (t ) = v C (t )
We get
•
1
x (t )
L 2
•
IS 1
1
x (t )
− x1 (t ) −
C C
RC 2
x1 (t ) =
x 2 (t ) =
We create function m-file containing the above differential equations.
MATLAB Script
% Solution of second-order differential equation
% The function diff2(x,y) is created to evaluate the diff. equation
% the name of the m-file is diff2.m
% the function is defined as:
%
function xdot = diff2(t,x)
is = 2;
c = 50e-6; L = 1/32; r = 10;
k1 = 1/c ; % 1/C
k2 = 1/L ; % 1/L
k3 = 1/(r*c); % 1/RC
xdot(1) = k2*x(2);
xdot(2) = k1*is - k1*x(1) - k3*x(2);
end
© 1999 CRC Press LLC
To simulate the differential equation defined in diff2 in the interval 0 ≤ t ≤ 30
ms, we note that
x1 (0) = i L (0) = 0 V
x 2 (0) = v C (0) = 20
Using the MATLAB ode23 function, we get
% solution of second-order differential equation
% the function diff2(x,y) is created to evaluate
% the differential equation
% the name of m-file is diff2.m
%
% Transient analysis of RLC circuit using ode function
% numerical solution
t0 = 0;
tf = 30e-3;
x0 = [0 20]; % Initial conditions
[t,x] = ode23('diff2',t0,tf,x0);
% Second column of matrix x represent capacitor voltage
subplot(211), plot(t,x(:,2))
xlabel('Time, s'), ylabel('Capacitor voltage, V')
text(0.01, 7, 'State Variable Approach')
% Transient analysis of RLC circuit from Example 5.5
t2 =0:1e-3:30e-3;
vt = -6.667*exp(-1600*t2) + 26.667*exp(-400*t2);
subplot(212), plot(t2,vt)
xlabel('Time, s'), ylabel('Capacitor voltage, V')
text(0.01, 4.5, 'Results from Example 5.5')
The plot is shown in Figure 5.13.
© 1999 CRC Press LLC
Figure 5.13 Capacitor Voltage v 0 ( t ) Obtained from Both State
Variable Approach and Laplace Transform
The results from the state variable approach and those obtained from Example
5.5 are identical.
Example 5.8
v S (t ) = 5u(t ) where u(t ) is the unit step function and
R1 = R2 = R3 = 10 KΩ , C1 = C2 = 5 µF , and L = 10 H, find and plot
the voltage v 0 ( t ) within the intervals of 0 to 5 s.
For Figure 5.11, if
Solution
Using the element values and Equations (5.36) to (5.38), we have
dv1 ( t )
= −40v1 ( t ) + 20 v 2 ( t ) + 20Vs
dt
© 1999 CRC Press LLC
dv 2 ( t )
= 20v1 ( t ) − 20 v 2 ( t ) − i1 ( t )
dt
di1 ( t )
. v 2 ( t ) − 1000i1 ( t )
= 01
dt
We create an m-file containing the above differential equations.
MATLAB Script
%
% solution of a set of first order differential equations
% the function diff3(t,v) is created to evaluate
% the differential equation
% the name of the m-file is diff3.m
%
function vdot = diff3(t,v)
vdot(1) = -40*v(1) + 20*v(2) + 20*5;
vdot(2) = 20*v(1) - 20*v(2) - v(3);
vdot(3) = 0.1*v(2) -1000*v(3);
end
To obtain the output voltage in the interval of 0 ≤ t ≤ 5 s, we note that the
output voltage
v 0 (t ) = v1 (t ) − v 2 (t )
Note that at t < 0, the step signal is zero so
v 0 (0) = v 2 (0) = i1 (0) = 0
Using ode45 we get
% solution of a set of first-order differential equations
% the function diff3(t,v) is created to evaluate
% the differential equation
% the name of the m-file is diff3.m
%
% Transient analysis of RLC circuit using state
© 1999 CRC Press LLC
% variable approach
t0 = 0;
tf = 2;
x0 = [0 0 0]; % initial conditions
[t,x] = ode23('diff3', t0, tf, x0);
tt = length(t);
for i = 1:tt
vo(i) = x(i,1) - x(i,2);
end
plot(t, vo)
title('Transient analysis of RLC')
xlabel('Time, s'), ylabel('Output voltage')
The plot of the output voltage is shown in Figure 5.14.
10,000 Ohms
Figure 5.14 Output Voltage
© 1999 CRC Press LLC
10 V
PL
RL
VL
SELECTED BIBLIOGRAPHY
1.
MathWorks, Inc., MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Biran, A. and Breiner, M. MATLAB for Engineers, Addison-Wesley,
1995.
3.
Etter, D.M., Engineering Problem Solving with MATLAB,
Edition, Prentice Hall, 1997.
4.
Nilsson, J.W., Electric Circuits, 3rd Edition, Addison-Wesley
Publishing Company, 1990.
5.
Vlach, J.O., Network Theory and CAD, IEEE Trans. on Education,
Vol. 36, No. 1, Feb. 1993, pp. 23 - 27.
6.
Meader, D. A., Laplace Circuit Analysis and Active Filters, Prentice
Hall, New Jersey, 1991.
2nd
EXERCISES
5.1 If the switch is opened at t = 0, find
time interval 0 ≤ t ≤ 5 s.
20 kilohms
v 0 (t ) . Plot v 0 (t ) between the
10 kilohms
t=0
30V
40 kilohms
Figure P5.1 Figure for Exercise 5.1
© 1999 CRC Press LLC
1 microfarads
Vo(t)
5.2
The switch is close at t = 0; find i( t ) between the intervals 0 to 10
ms. The resistance values are in ohms.
16
i(t)
9V
8
8
4H
t=0
Figure P5.2 Figure for Exercise 5.2
5.3
For the series RLC circuit, the switch is closed at t = 0. The initial
energy in the storage elements is zero. Use MATLAB to find v 0 ( t ) .
10 Ohms
1.25 H
t=0
8V
0.25 microfarads
Vo(t)
Figure P5.3 Circuit for Exercise 5.3
5.4
Use MATLAB to solve the following differential equation
d 3 y(t )
d 2 y( t )
dy ( t )
+7
+ 14
+ 12 y ( t ) = 10
3
2
dt
dt
dt
© 1999 CRC Press LLC
with initial conditions
d 2 y(0)
=5
dt 2
dy ( 0)
= 2,
dt
y( 0) = 1 ,
Plot y(t) within the intervals of 0 and 10 s.
5.5 For Figure P5.5, if VS = 5u( t ), determine the voltages V1(t), V2(t),
V3(t) and V4(t) between the intervals of 0 to 20 s. Assume that the initial
voltage across each capacitor is zero.
1 kilohms
V1
VS
1 kilohms
V2
1pF
V3
1 kilohms
2pF
1 kilohms
3pF
Figure P5.5 RC Network
5.6
For the differential equation
d 2 y( t )
dy ( t )
+5
+ 6 y ( t ) = 3 sin( t ) + 7 cos( t )
2
dt
dt
with initial conditions
(a) Determine
y( 0) = 4 and
y (t ) using Laplace transforms.
(b) Use MATLAB to determine
(c) Sketch
dy ( 0)
= −1
dt
y (t ) .
y (t ) obtained in parts (a) and (b).
(d) Compare the results obtained in part c.
© 1999 CRC Press LLC
V4
4pF
CHAPTER SIX
AC ANALYSIS AND NETWORK FUNCTIONS
This chapter discusses sinusoidal steady state power calculations. Numerical
integration is used to obtain the rms value, average power and quadrature
power. Three-phase circuits are analyzed by converting the circuits into the
frequency domain and by using the Kirchoff voltage and current laws. The unknown voltages and currents are solved using matrix techniques.
Given a network function or transfer function, MATLAB has functions that can
be used to (i) obtain the poles and zeros, (ii) perform partial fraction expansion, and (iii) evaluate the transfer function at specific frequencies. Furthermore, the frequency response of networks can be obtained using a MATLAB
function. These features of MATLAB are applied in this chapter.
6.1
STEADY STATE AC POWER
Figure 6.1 shows an impedance with voltage across it given by
rent through it
v (t ) and cur-
i (t ) .
i(t)
v(t)
Z
+
Figure 6.1 One-Port Network with Impedance Z
The instantaneous power
p(t ) is
p(t ) = v (t )i (t )
(6.1)
If v ( t ) and i ( t ) are periodic with period T, the rms or effective values of
the voltage and current are
© 1999 CRC Press LLC
T
Vrms =
1
v 2 ( t )dt
T ∫0
I rms =
1 2
i ( t )dt
T ∫0
(6.2)
T
(6.3)
where
Vrms is the rms value of v (t )
I rms is the rms value of i (t )
The average power dissipated by the one-port network is
T
P=
1
v ( t )i( t )dt
T ∫0
The power factor,
pf =
(6.4)
pf , is given as
P
Vrms I rms
(6.5)
For the special case, where both the current
sinusoidal, that is,
i (t ) and voltage v (t ) are both
v (t ) = Vm cos( wt + θV )
(6.6)
i (t ) = I m cos( wt + θI )
(6.7)
and
the rms value of the voltage
Vrms =
Vm
2
and that of the current is
© 1999 CRC Press LLC
v (t ) is
(6.8)
I rms =
Im
2
(6.9)
P is
The average power
P = Vrms I rms cos(θV − θ I )
The power factor,
(6.10)
pf , is
pf = cos(θV − θI )
The reactive power
(6.11)
Q is
Q = Vrms I rms sin(θV − θI )
(6.12)
and the complex power, S, is
S = P + jQ
(6.13)
[
S = Vrms I rms cos(θV − θI ) + j sin(θV − θI )
]
(6.14)
Equations (6.2) to (6.4) involve the use of integration in the determination of
the rms value and the average power. MATLAB has two functions, quad and
quad8, for performing numerical function integration.
6.1.1
MATLAB Functions quad and quad8
The quad function uses an adaptive, recursive Simpson’s rule. The quad8
function uses an adaptive, recursive Newton Cutes 8 panel rule. The quad8
function is better than the quad at handling functions with “soft” singularities
such as
∫
xdx . Suppose we want to find q given as
b
q = ∫ funct ( x )dx
a
The general forms of quad and quad8 functions that can be used to find q are
© 1999 CRC Press LLC
quad (' funct ', a , b, tol , trace)
quad 8(' funct ' , a , b, tol , trace)
where
funct
is a MATLAB function name (in quotes) that returns a
vector of values of f ( x ) for a given vector of input values
x.
a
is the lower limit of integration.
b
is the upper limit of integration.
tol
is the tolerance limit set for stopping the iteration of the
numerical integration. The iteration continues until the relative error is less than tol. The default value is 1.0e-3.
trace
allows the plot of a graph showing the process of the
numerical integration. If the trace is nonzero, a graph is
plotted. The default value is zero.
Example 6.1 shows the use of the quad function to perform alternating current
power calculations.
Example 6.1
v (t ) = 10 cos(120πt + 30 0 ) and
i (t ) = 6 cos(120πt + 60 0 ) . Determine the average power, rms value of
v (t ) and the power factor using (a) analytical solution and (b) numerical so-
For Figure 6.1, if
lution.
Solution
MATLAB Script
diary ex6_1.dat
% This program computes the average power, rms value and
% power factor using quad function. The analytical and
% numerical results are compared.
% numerical calculations
© 1999 CRC Press LLC
T = 2*pi/(120*pi); % period of the sin wave
a = 0; % lower limit of integration
b = T; % upper limit of integration
x = 0:0.02:1;
t = x.*b;
v_int = quad('voltage1', a, b);
v_rms = sqrt(v_int/b); % rms of voltage
i_int = quad('current1',a,b);
i_rms = sqrt(i_int/b); % rms of current
p_int = quad('inst_pr', a, b);
p_ave = p_int/b; % average power
pf = p_ave/(i_rms*v_rms); % power factor
%
% analytical solution
%
p_ave_an = (60/2)*cos(30*pi/180); % average power
v_rms_an = 10.0/sqrt(2);
pf_an = cos(30*pi/180);
% results are printed
fprintf('Average power, analytical %f \n Average power, numerical:
%f \n', p_ave_an,p_ave)
fprintf('rms voltage, analytical: %f \n rms voltage, numerical: %f \n',
v_rms_an, v_rms)
fprintf('power factor, analytical: %f \n power factor, numerical: %f \n',
pf_an, pf)
diary
The following functions are used in the above m-file:
function vsq = voltage1(t)
% voltage1 This function is used to
%
define the voltage function
vsq = (10*cos(120*pi*t + 60*pi/180)).^2;
end
function isq = current1(t)
% current1 This function is to define the current
%
isq = (6*cos(120*pi*t + 30.0*pi/180)).^2;
end
© 1999 CRC Press LLC
function pt = inst_pr(t)
% inst_pr This function is used to define
%
instantaneous power obtained by multiplying
%
sinusoidal voltage and current
it = 6*cos(120*pi*t + 30.0*pi/180);
vt = 10*cos(120*pi*t + 60*pi/180);
pt = it.*vt;
end
The results obtained are
Average power, analytical 25.980762
Average power, numerical: 25.980762
rms voltage, analytical: 7.071068
rms voltage, numerical: 7.071076
power factor, analytical: 0.866025
power factor, numerical: 0.866023
From the results, it can be seen that the two techniques give almost the same
answers.
6.2
SINGLE- AND THREE-PHASE AC CIRCUITS
Voltages and currents of a network can be obtained in the time domain. This
normally involves solving differential equations. By transforming the differential equations into algebraic equations using phasors or complex frequency
representation, the analysis can be simplified. For a voltage given by
v (t ) = Vm eσt cos( wt + θ )
v(t ) = Re[Vm e σt cos( wt + θ )]
(6.15)
the phasor is
V = Vm e jθ = Vm ∠θ
and the complex frequency
© 1999 CRC Press LLC
s is
(6.16)
s = σ + jw
(6.17)
When the voltage is purely sinusoidal, that is
v 2 (t ) = Vm2 cos( wt + θ2 )
(6.18)
then the phasor
V2 = Vm2 e jθ2 = Vm2 ∠θ2
(6.19)
and complex frequency is purely imaginary, that is,
s = jw
(6.20)
To analyze circuits with sinusoidal excitations, we convert the circuits into
the s-domain with s = jw . Network analysis laws, theorems, and rules are
used to solve for unknown currents and voltages in the frequency domain. The
solution is then converted into the time domain using inverse phasor transformation. For example, Figure 6.2 shows an RLC circuit in both the time and
frequency domains.
C1
R1
L1
L2
Vs(t) = 8 cos (10t + 15 o) V
R2
(a)
© 1999 CRC Press LLC
R3
V3(t)
1/(j10C1)
R1
j10 L1
j10 L2
V2
V1
Vs = 8 15o
R2
R3
V3
(b)
Figure 6.2 RLC Circuit with Sinusoidal Excitation (a) Time
Domain (b) Frequency Domain Equivalent
R1 , R2 , R3 , L1 , L2 and C1 are known, the voltage V3 can
be obtained using circuit analysis tools. Suppose V3 is
If the values of
V3 = Vm 3 ∠θ3 ,
then the time domain voltage V3 (t) is
v 3 (t ) = Vm3 cos( wt + θ3 )
The following two examples illustrate the use of MATLAB for solving onephase circuits.
Example 6.2
R1 = 20 Ω, R2 = 100Ω , R3 = 50 Ω , and L1 = 4 H, L2 =
8 H and C1 = 250µF, find v 3 ( t ) when w = 10 rad/s.
In Figure 6.2, if
Solution
Using nodal analysis, we obtain the following equations.
At node 1,
© 1999 CRC Press LLC
V1 − Vs V1 − V2
V − V3
+
+ 1
=0
1
R1
j10 L1
( j10C1 )
(6.21)
At node 2,
V2 − V1 V2 V2 − V3
+
+
=0
j10 L1 R2
j10 L2
(6.22)
At node 3,
V3 V3 − V2
V − V1
+
+ 3
=0
1
R3
j10 L2
( j10C1 )
(6.23)
Substituting the element values in the above three equations and simplifying,
we get the matrix equation
0.05 − j 0.0225
− j 0.0025 V1
j 0.025
0.01 − j 0.0375
j 0.025
j 0.0125 V2 =
− j 0.0025
0.02 − j 0.01 V3
j 0.0125
0.4∠150
0
0
The above matrix can be written as
[Y ][V ] = [ I ] .
We can compute the vector [v] using the MATLAB command
V = inv(Y ) * I
where
inv(Y ) is the inverse of the matrix [Y ] .
A MATLAB program for solving V3 is as follows:
MATLAB Script
diary ex6_2.dat
% This program computes the nodal voltage v3 of circuit Figure 6.2
© 1999 CRC Press LLC
% Y is the admittance matrix; % I is the current matrix
% V is the voltage vector
Y = [0.05-0.0225*j 0.025*j
-0.0025*j;
0.025*j
0.01-0.0375*j 0.0125*j;
-0.0025*j
0.0125*j
0.02-0.01*j];
c1 = 0.4*exp(pi*15*j/180);
I = [c1
0
0]; % current vector entered as column vector
V = inv(Y)*I; % solve for nodal voltages
v3_abs = abs(V(3));
v3_ang = angle(V(3))*180/pi;
fprintf('voltage V3, magnitude: %f \n voltage V3, angle in degree:
%f', v3_abs, v3_ang)
diary
The following results are obtained:
voltage V3, magnitude: 1.850409
voltage V3, angle in degree: -72.453299
From the MATLAB results, the time domain voltage
v 3 (t ) is
. cos(10t − 72.450 ) V
v 3 (t ) = 185
Example 6.3
For the circuit shown in Figure 6.3, find the current
v C (t ) .
© 1999 CRC Press LLC
i1 (t ) and the voltage
8mH
400 microfarads
4 Ohms
i(t)
10 Ohms
5 mH
5 cos (103t) V
2 cos (103 t + 75 o) V
6 Ohms
Vc(t)
100 microfarads
Figure 6.3 Circuit with Two Sources
Solution
Figure 6.3 is transformed into the frequency domain. The resulting circuit is
shown in Figure 6.4. The impedances are in ohms.
4
-j2.5
j8
j5
I1
5 0o V
10
I2
6
Vc
2 75 o V
-j10
Figure 6.4 Frequency Domain Equivalent of Figure 6.3
Using loop analysis, we have
− 5∠0 0 + (4 − j 2.5) I 1 + (6 + j5 − j10)( I 1 − I 2 ) = 0 (6.24)
(10 + j8) I 2 + 2 ∠750 + (6 + j5 − j10)( I 2 − I 1 ) = 0
Simplifying, we have
© 1999 CRC Press LLC
(6.25)
(10 − j 7.5) I 1 − (6 − j5) I 2 = 5∠0 0
− (6 − j5) I 1 + (16 + j 3) I 2 = −2∠750
In matrix form, we obtain
10 − j 7.5 −6 + j5 I1 5∠0 0
−6 + j5 16 + j3 I =
0
2 −2∠75
The above matrix equation can be rewritten as
[ Z ][ I ] = [V ] .
We obtain the current vector
[I]
using the MATLAB command
I = inv( Z ) * V
where
inv( Z ) is the inverse of the matrix [ Z ] .
The voltage
VC can be obtained as
VC = ( − j10)( I 1 − I 2 )
A MATLAB program for determining
I 1 and Va is as follows:
MATLAB Script
diary ex6_3.dat
% This programs calculates the phasor current I1 and
% phasor voltage Va.
% Z is impedance matrix
% V is voltage vector
% I is current vector
Z = [10-7.5*j -6+5*j;
-6+5*j 16+3*j];
b = -2*exp(j*pi*75/180);
© 1999 CRC Press LLC
V = [5
b]; % voltage vector in column form
I = inv(Z)*V; % solve for loop currents
i1 = I(1);
i2 = I(2);
Vc = -10*j*(i1 - i2);
i1_abs = abs(I(1));
i1_ang = angle(I(1))*180/pi;
Vc_abs = abs(Vc);
Vc_ang = angle(Vc)*180/pi;
%results are printed
fprintf('phasor current i1, magnitude: %f \n phasor current i1, angle in
degree: %f \n', i1_abs,i1_ang)
fprintf('phasor voltage Vc, magnitude: %f \n phasor voltage Vc, angle
in degree: %f \n',Vc_abs,Vc_ang)
diary
The following results were obtained:
phasor current i1, magnitude: 0.387710
phasor current i1, angle in degree: 15.019255
phasor voltage Vc, magnitude: 4.218263
phasor voltage Vc, angle in degree: -40.861691
The current
i1 (t ) is
i1 (t ) = 0.388 cos(10 3 t + 15.02 0 ) A
and the voltage
v C (t ) is
v C (t ) = 4.21cos(10 3 t − 40.86 0 ) V
Power utility companies use three-phase circuits for the generation, transmission and distribution of large blocks of electrical power. The basic structure of
a three-phase system consists of a three-phase voltage source connected to a
three-phase load through transformers and transmission lines. The three-phase
voltage source can be wye- or delta-connected. Also the three-phase load can
be delta- or wye-connected. Figure 6.5 shows a 3-phase system with wyeconnected source and wye-connected load.
© 1999 CRC Press LLC
Van
Z T1
Vbn
Z T2
Vcn
Z T3
Z Y1
Z t4
Z Y3
Z Y2
Figure 6.5 3-phase System, Wye-connected Source and Wyeconnected Load
Van
Z t1
Vbn
Z t2
Z
1
Z
Vcn
Z t3
Z
3
2
Figure 6.6 3-phase System, Wye-connected Source and Deltaconnected Load
For a balanced abc system, the voltages Van , Vbn , Vcn have the same magnitude and they are out of phase by 1200. Specifically, for a balanced abc system, we have
Van = V P ∠0 0
Vbn = V P ∠ − 120 0
Vcn = V P ∠120 0
© 1999 CRC Press LLC
(6.26)
For cba system
Van = V P ∠0 0
Vbn = V P ∠120 0
Vcn = V P ∠ − 120 0
(6.27)
The wye-connected load is balanced if
ZY 1 = Z Y 2 = Z Y 3
(6.28)
Similarly, the delta-connected load is balanced if
Z ∆1 = Z ∆ 2 = Z ∆ 3
(6.29)
We have a balanced three-phase system of Equations (6.26) to (6.29) that are
satisfied with the additional condition
ZT1 = ZT 2 = ZT 3
(6.30)
Analysis of balanced three-phase systems can easily be done by converting the
three-phase system into an equivalent one-phase system and performing simple
hand calculations. The method of symmetrical components can be used to analyze unbalanced three-phase systems. Another method that can be used to analyze three-phase systems is to use KVL and KCL. The unknown voltage or
currents are solved using MATLAB. This is illustrated by the following example.
Example 6.4
In Figure 6.7, showing an unbalanced wye-wye system, find the phase voltages V AN , V BN and VCN .
Solution
Using KVL, we can solve for
I 1 , I 2 , I 3 . From the figure, we have
110∠0 0 = (1 + j1) I 1 + (5 + j12) I 1
© 1999 CRC Press LLC
(6.31)
110∠ − 120 0 = (1 − j 2) I 2 + (3 + j 4) I 2
(6.32)
110∠120 0 = (1 − j 0.5) I 3 + (5 − j12) I 3
(6.33)
110 0o V
+
1 + j1 Ohms
5 + j12 Ohms
A
N
I1
o
-
110 -120 V
+
1 - j2 Ohms
3 + j4 Ohms
B
I2
o
110 120 V
+
5 - j12 Ohms
1 - j0.5 Ohms
C
I3
Figure 6.7 Unbalanced Three-phase System
Simplifying Equations (6.31), (6.32) and (6.33), we have
110∠0 0 = (6 + j13) I 1
(6.34)
110∠ − 120 0 = (4 + j 2) I 2
(6.35)
110∠120 0 = (6 − j12.5) I 3
(6.36)
and expressing the above three equations in matrix form, we have
6 + j13
0
0 I1
4 + j2
0 I 2 =
0
0
0
6 − j12.5 I 3
The above matrix can be written as
[ Z ][ I ] = [V ]
© 1999 CRC Press LLC
110∠0 0
0
110∠ − 120
110∠120 0
We obtain the vector
[I]
using the MATLAB command
I = inv ( Z ) * V
The phase voltages can be obtained as
V AN = (5 + j12) I 1
VBN = (3 + j 4) I 2
VCN = (5 − j12)( I 3 )
The MATLAB program for obtaining the phase voltages is
MATLAB Script
diary ex6_4.dat
% This program calculates the phasor voltage of an
% unbalanced three-phase system
% Z is impedance matrix
% V is voltage vector and
% I is current vector
Z = [6-13*j 0
0;
0
4+2*j 0;
0
0
6-12.5*j];
c2 = 110*exp(j*pi*(-120/180));
c3 = 110*exp(j*pi*(120/180));
V = [110; c2; c3]; % column voltage vector
I = inv(Z)*V; % solve for loop currents
% calculate the phase voltages
%
Van = (5+12*j)*I(1);
Vbn = (3+4*j)*I(2);
Vcn = (5-12*j)*I(3);
Van_abs = abs(Van);
Van_ang = angle(Van)*180/pi;
Vbn_abs = abs(Vbn);
Vbn_ang = angle(Vbn)*180/pi;
Vcn_abs = abs(Vcn);
Vcn_ang = angle(Vcn)*180/pi;
% print out results
© 1999 CRC Press LLC
fprintf('phasor voltage Van,magnitude: %f \n phasor voltage Van, angle in degree: %f \n', Van_abs, Van_ang)
fprintf('phasor voltage Vbn,magnitude: %f \n phasor voltage Vbn, angle in degree: %f \n', Vbn_abs, Vbn_ang)
fprintf('phasor voltage Vcn,magnitude: %f \n phasor voltage Vcn, angle in degree: %f \n', Vcn_abs, Vcn_ang)
diary
The following results were obtained:
phasor voltage Van,magnitude: 99.875532
phasor voltage Van, angle in degree: 132.604994
phasor voltage Vbn,magnitude: 122.983739
phasor voltage Vbn, angle in degree: -93.434949
phasor voltage Vcn,magnitude: 103.134238
phasor voltage Vcn, angle in degree: 116.978859
6.3
NETWORK CHARACTERISTICS
Figure 6.8 shows a linear network with input x ( t ) and output
complex frequency representation is also shown.
linear
network
x(t)
y (t ) . Its
y(t)
(a)
X(s)est
linear
network
Y(s)est
(b)
Figure 6.8 Linear Network Representation (a) Time Domain
(b) s- domain
In general, the input
equation
© 1999 CRC Press LLC
x (t ) and output y (t ) are related by the differential
an
d n y(t )
d n−1 y ( t )
dy ( t )
+ a n−1
+ !+ a1
+ a0 y ( t ) =
n
n −1
dt
dt
dt
d m x( t )
d m−1 x( t )
dx( t )
+
+" b1
+ b0 x( t )
bm
b
m−1
dt m
dt m−1
dt
(6.37)
where
a n , a n −1 , ..., a 0 , bm , bm−1 , ... b0 are real constants.
x (t ) = X ( s)e st , then the output must have the form y (t ) = Y ( s)e st ,
where X ( s) and Y ( s) are phasor representations of x ( t ) and y ( t ) . From
If
equation (6.37), we have
( a n s n + a n−1s n−1 + " + a1s + a 0 )Y ( s )e st =
( bm s m + bm−1s m−1 + " + b1s + b0 ) X ( s )e st
(6.38)
and the network function
H ( s) =
Y ( s ) bm s m + bm−1s m−1 + " b1s + b0
=
X ( s ) a n s n + a n −1s n −1 + " a1s + a0
(6.39)
The network function can be rewritten in factored form
H ( s) =
k ( s − z1 )( s − z 2 ) " ( s − z m )
( s − p1 )( s − p2 ) " ( s − pn )
(6.40)
where
k is a constant
z1 , z 2 , ..., z m are zeros of the network function.
p1 , p2 , ..., pn are poles of the network function.
The network function can also be expanded using partial fractions as
H ( s) =
© 1999 CRC Press LLC
r1
r2
r
+
+ .... + n + k ( s)
s − p1 s − p2
s − pn
(6.41)
6.3.1
MATLAB functions roots, residue and polyval
MATLAB has the function roots that can be used to obtain the poles and zeros
of a network function. The MATLAB function residue can be used for partial
fraction expansion. Furthermore, the MATLAB function polyval can be used
to evaluate the network function.
The MATLAB function roots determines the roots of a polynomial. The general form of the roots function is
r = roots( p)
(6.42)
where
p is a vector containing the coefficients of the polynomial in
descending order
r is a column vector containing the roots of the polynomials
For example, given the polynomial
f ( x ) = x 3 + 9 x 2 + 23x + 15
the commands to compute and print out the roots of
f ( x ) are
p = [1 9 23 15]
r = roots (p)
and the values printed are
r =
-1.0000
-3.0000
-5.0000
Given the roots of a polynomial, we can obtain the coefficients of the polynomial by using the MATLAB function poly
Thus
S = poly ( [ -1
© 1999 CRC Press LLC
-3
-5 ]1 )
(6.43)
will give a row vector s given as
S=
1.0000
9.0000
23.0000
15.0000
The coefficients of S are the same as those of p.
The MATLAB function polyval is used for polynomial evaluation. The general form of polyval is
polyval ( p, x )
(6.44)
where
p is a vector whose elements are the coefficients of a polynomial in
descending powers
polyval ( p, x ) is the value of the polynomial evaluated at x
For example, to evaluate the polynomial
f ( x ) = x 3 − 3x 2 − 4 x + 15
at x = 2 , we use the command
p = [1 -3 -4
polyval(p, 2)
15];
Then we get
ans =
3
The MATLAB function residue can be used to perform partial fraction expansion. Assuming H ( s) is the network function, since H ( s) may represent
an improper fraction, we may express
H ( s) =
© 1999 CRC Press LLC
B ( s)
A( s)
H ( s) as a mixed fraction
(6.45)
N
H ( s) = ∑ k n s n +
n=0
N ( s)
D ( s)
(6.46)
where
N ( s)
is a proper fraction
D ( s)
From equations (6.41) and ( 6.46), we get
N
r1
r2
rn
H( s) =
+
+ .... +
+ ∑ kn sn
s − p1 s − p2
s − pn n=0
(6.47)
Given the coefficients of the numerator and denominator polynomials, the
MATLAB residue function provides the values of r1, r2, ...... rn , p1, p2, .....pn,
an d k1, k2 , .....kn . The general form of the residue function is
[r , p, k ] = residue(num, den)
(6.48)
where
num
is a row vector whose entries are the coefficients of the
numerator polynomial in descending order
den
is a row vector whose entries are the coefficient of the
denominator polynomial in descending order
r
is returned as a column vector
p
(pole locations) is returned as a column vector
k
(direct term) is returned as a row vector
The command
[num, den] = residue(r , p, k )
© 1999 CRC Press LLC
(6.49)
Converts the partial fraction expansion back to the polynomial ratio
H ( s) =
B ( s)
A( s)
For example, given
H ( s) =
4 s 4 + 3s 3 + 6s 2 + 10s + 20
s 4 + 2 s 3 + 5s 2 + 2 s + 8
(6.50)
for the above network function, the following commands will perform partial
fraction expansion
num = [4 3 6 10 20];
den = [1 2 5 2 8];
[r, p, k] = residue(num, den)
(6.51)
and we shall get the following results
r=
-1.6970 + 3.0171i
-1.6970 - 3.0171i
-0.8030 - 0.9906i
-0.8030 + 0.9906i
p=
-1.2629 + 1.7284i
-1.2629 - 1.7284i
0.2629 + 1.2949i
0.2629 - 1.2949i
k=
4
The following two examples show how to use MATLAB function roots to
find poles and zeros of circuits.
Example 6.5
For the circuit shown below, (a) Find the network function
© 1999 CRC Press LLC
H ( s) =
Vo ( s)
V S ( s)
H ( s) , and
cos(2t + 40 0 ) , find v 0 (t ) .
(b) Find the poles and zeros of
(c) if
v S (t ) = 10e −3t
6 Ohms
3H
Vs(t)
2 Ohms
Vo(t)
4H
Figure 6.9 Circuit for Example 6.5
Solution
In the s-domain, the above figure becomes
3s
Vs
6
2
4s
Vo(s)
Figure 6.10 S-domain Equivalent Circuit of Figure 6.9
[2 (6 + 4 s)]
V0 ( s) V0 ( s) V X ( s)
4s
=
=
VS ( s) V X ( s) VS ( s) (6 + 4 s) (2 (6 + 4 s)) + 3s
[
]
Simplifying, we get
V0 ( s)
4 s 2 + 6s
= 3
VS ( s) 6s + 25s 2 + 30s + 9
© 1999 CRC Press LLC
(6.52)
The phasor voltage VS
= 10∠40 o ; s = −3 + j 2
V0 ( s) = (10∠40 o ) H ( s)
s =−3+ j 2
(b, c) MATLAB is used to find the poles, zeros and
v 0 (t ) .
MATLAB Script
diary ex6_5.dat
% Program for poles and zeros
num = [4 6 0];
den = [6 25 30 9];
disp('the zeros are')
z = roots(num)
disp('the poles are')
p = roots(den)
% program to evaluate transfer function and
% find the output voltage
s1 = -3+2*j;
n1 = polyval(num,s1);
d1 = polyval(den,s1);
vo = 10.0*exp(j*pi*(40/180))*n1/d1;
vo_abs = abs(vo);
vo_ang = angle(vo)*180/pi;
% print magnitude and phase of output voltage
fprintf('phasor voltage vo, magnitude: %f \n phasor voltage vo, angle
in degrees: %f', vo_abs, vo_ang)
diary
MATLAB results are
Zeros
z=
0
-1.5000
Poles
p=
-2.2153
-1.5000
-0.4514
phasor voltage vo, magnitude: 3.453492
© 1999 CRC Press LLC
phasor voltage vo, angle in degrees: -66.990823
From the results, the output voltage is given as
v (t ) = 3.45e −3t cos(2t − 66.99 0 )
Example 6.6
Find the inverse Laplace transform of
G ( s) =
10s 2 + 20s + 40
s 3 + 12 s 2 + 47 s + 60
Solution
MATLAB Script
diary ex6_6.dat
% MATLAB is used to do the partial fraction expansion
%
num = [10 20 40];
den = [1 12 47 60];
% we get the following results
[r, p, k] = residue(num,den)
diary
MATLAB results are
r=
95.0000
-120.0000
35.0000
p=
-5.0000
-4.0000
-3.0000
k=
© 1999 CRC Press LLC
[]
From the results, we get
G ( s) =
95
120
35
−
+
s+5 s+4 s+3
and the inverse Laplace transform is
g (t ) = 35e −3t − 120e −4 t + 95e −5t
6.4
(6.53)
FREQUENCY RESPONSE
The general form of a transfer function of an analog circuit is given in Equation (6.39). It is repeated here.
H ( s) =
Y ( s ) bm s m + bm−1s m−1 + " b1s + b0
=
X ( s ) a n s n + a n −1s n −1 + " a1s + a0
More specifically, for a second-order analog filter, the following transfer functions can be obtained:
(i) Lowpass
k1
s + Bs + w02
(6.54)
k2 s2
H HP ( s) = 2
s + Bs + w02
(6.55)
H LP ( s) =
2
(ii) Highpass
(iii) Bandpass
H BP ( s) =
(iv) Bandreject
© 1999 CRC Press LLC
k3s
s + Bs + w02
2
(6.56)
H BR ( s) =
k4 s2 + k5
s 2 + Bs + w02
(6.57)
where
k1 , k 2 , k 3 , k 4 , B and w0 are constants
Figure 6.11 shows the circuit diagram of some filter sections.
C
R
R
Vo
Vs
C
(K - 1)Rf
Rf
(a)
R
C
C
Vo
Vs
R
(K - 1)Rf
Rf
(b)
© 1999 CRC Press LLC
C
R1
R3
C
Vs
V0
R2
(c )
Figure 6.11 Active Filters (a) Lowpass, (b) Highpass and
(c ) Bandpass
Frequency response is the response of a network to sinusoidal input signal. If
we substitute s = jw in the general network function, H ( s), we get
H ( s)
s = jw
= M ( w) ∠θ ( w)
(6.58)
where
M ( w) = H ( jw)
(6.59)
θ ( w) = ∠H ( jw)
(6.60)
and
The plot of M (ω) versus ω is the magnitude characteristics or response. Also,
the plot of θ ( w) versus ω is the phase response. The magnitude and phase
characteristics can be obtained using MATLAB function freqs.
© 1999 CRC Press LLC
6.4.1
MATLAB function freqs
MATLAB function freqs is used to obtain the frequency response of transfer
function H ( s) . The general form of the frequency function is
hs = freqs(num, den, range)
(6.61)
where
Y ( s ) bm s m + bm−1s m−1 + " b1s + b0
=
X ( s ) a n s n + a n −1s n −1 + " a1s + a0
H ( s) =
[
num = bm b.m−1 ... b1 b0
]
den = [a n a n −1 ... a1 a 0 ]
range
is range of frequencies for case
hs
is the frequency response (in complex number form)
(6.62)
(6.63)
(6.64)
Suppose we want to graph the frequency response of the transfer function
given as
H ( s) =
2s2 + 4
s 2 + 4 s + 16
(6.65)
We can use the following commands to find the magnitude characteristics
num = [2 0 4];
den = [1 4 16];
w = logspace(-2, 4);
h = freqs(num, den, w);
f = w/(2*pi);
mag = 20*log10(abs(h));
semilogx(f, mag)
title('Magnitude Response')
xlabel('Frequency, Hz')
ylabel('Gain, dB')
© 1999 CRC Press LLC
The frequency response is shown in Figure 6.12.
Figure 6.12 Magnitude Response of Equation (6.65)
The following example shows how to obtain and plot the frequency response
of an RLC circuit.
Example 6.7
For the RLC circuit shown in Figure 6.13, (a) show that the transfer function is
R
V ( s)
L
H ( s) = o
=
1
R
Vi ( s)
s2 + s +
L LC
s
(6.66)
(b) If L = 5 H, C = 1.12 µF, and R = 10000 Ω, plot the frequency response.
(c) What happens when R = 100 Ω, but L and C remain unchanged?
© 1999 CRC Press LLC
L
C
Vi
R
Vo(t)
Figure 6.13 RLC Circuit
Solution
(a) In the frequency domain,
H ( s) =
V0 ( s)
=
Vi ( s)
R
1
R + sL +
sC
=
which is
R
V0 ( s)
L
=
H ( s) =
1
R
Vi ( s)
s2 + s +
L LC
s
Parts (b) and (c ) are solved using MATLAB.
MATLAB Script
% Frequency response of RLC filter
%
l = 5;
c = 1.25e-6;
r1 = 10000;
r2 = 100;
num1 = [r1/l 0];
den1 = [1 r1/l 1/(l*c)];
w = logspace(1,4);
h1 = freqs(num1,den1,w);
© 1999 CRC Press LLC
sCR
s LC + sCR + 1
2
(6.67)
f = w/(2*pi);
mag1 = abs(h1);
phase1 = angle(h1)*180/pi;
num2 = [r2/l 0];
den2 = [1 r2/l 1/(l*c)];
h2 = freqs(num2,den2,w);
mag2 = abs(h2);
phase2 = angle(h2)*180/pi;
% Plot the response
subplot(221), loglog(f, mag1,'.')
title('magnitude response R=10K')
ylabel('magnitude')
subplot(222), loglog(f,mag2,'.')
title('magnitude response R=.1K')
ylabel('magnitude')
subplot(223), semilogx(f, phase1,'.')
title('phase response R=10K'),...
xlabel('Frequency, Hz'), ylabel('angle in degrees')
subplot(224), semilogx(f, phase2,'.')
title('phase response R=.1K'),...
xlabel('Frequency, Hz'), ylabel('angle in degrees')
The plots are shown in Figure 6.14. As the resistance is decreased from
10,000 to 100 Ohms, the bandwidth of the frequency response decreases and
the quality factor of the circuit increases.
© 1999 CRC Press LLC
Figure 6.14 Frequency Response of an RLC Circuit
SELECTED BIBLIOGRAPHY
© 1999 CRC Press LLC
1.
MathWorks, Inc., MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Biran, A. and Breiner, M., MATLAB for Engineers, AddisonWesley, 1995.
3.
Etter, D.M., Engineering Problem Solving with MATLAB, 2nd
Edition, Prentice Hall, 1997.
4.
Nilsson, J.W. , Electric Circuits, 3rd Edition, Addison-Wesley
Publishing Company, 1990.
5.
Vlach, J.O., Network Theory and CAD, IEEE Trans. on Education,
Vol. 36, Feb. 1993, pp. 23-27.
6.
Meader, D.A., Laplace Circuit Analysis and Active Filters,
Prentice Hall, New Jersey, 1991.
7.
Johnson, D. E. Johnson, J.R. and Hilburn, J.L., Electric Circuit
Analysis, 3rd Edition, Prentice Hall, New Jersey, 1997.
EXERCISES
6.1
If
v (t ) is periodic with one period of v (t ) given as
v (t ) = 16(1 − e −6t ) V 0 ≤ t < 2 s
(a) Use MATLAB to find the rms value of
v (t )
(b) Obtain the rms value of v ( t ) using analytical technique.
Compare your result with that obtained in part (a).
(c) Find the power dissipated in the 4-ohm resistor when the
voltage v ( t ) is applied across the 4-ohm resistor.
v(t)
4 Ohms
R
Figure P6.1 Resistive Circuit for part (c)
6.2
A balanced Y-Y positive sequence system has phase voltage of the
Van = 120∠0 0 rms if the load impedance per phase is
(11 + j 4.5) Ω, and the transmission line has an impedance per phase
of (1 + j 0.5) Ω.
source
(a)
(b)
© 1999 CRC Press LLC
Use analytical techniques to find the magnitude of the line
current, and the power delivered to the load.
Use MATLAB to solve for the line current and the power
delivered to the load.
(c )
6.3
Compare the results of parts (a) and (b).
For the unbalanced 3-phase system shown in Figure P6.3, find the
currents I1 , I 2 , I 3 and hence I bB . Assume that Z A = 10 + j 5 Ω,
Z B = 15 + j 7 Ω and ZC = 12 − j 3 Ω .
120 0o V rms
A
a
1 Ohm
I1
ZA
I3
120 -120 o V rms
b
B
ZC
2 Ohms
I2
120
ZB
120 o V rms
c
C
1 Ohm
Figure P6.3 Unbalanced Three-phase System
6.4
For the system with network function
H ( s) =
s 3 + 4 s 2 + 16s + 4
s 4 + 20s 3 + 12 s 2 + s + 10
find the poles and zeros of
6.5
© 1999 CRC Press LLC
H ( s).
Use MATLAB to determine the roots of the following polynomials.
Plot the polynomial over the appropriate interval to verify the roots
location.
(a)
f 1 ( x) = x 2 + 4 x + 3
(b)
f 2 ( x ) = x 3 + 5x 2 + 9 x + 5
f 3 ( x ) = 2 x 5 − 4 x 4 − 12 x 3 + 27 x 2 + 8 x − 16
(c)
6.6
Vo ( s)
20s
,
=
2
Vi ( s) 15s + 23s + 16
If
find
6.7
v 0 (t ) given that vi (t ) = 2.3e −2 t cos(5t + 30 0 ) .
For the circuit of Figure P6.7
(a)
Find the transfer function
(b)
If
Vo ( s)
.
Vi ( s)
vi (t ) = 10e −5t cos(t + 10 0 ) , find v 0 (t ) .
2 Ohms
Vi(t)
2H
0.5 F
4 Ohms
Figure P6.7 RLC Circuit
6.8
For Figure P6.8,
(a) Find the transfer function
H ( s) =
Vo ( s)
.
Vi ( s)
(b) Use MATLAB to plot the magnitude characteristics.
© 1999 CRC Press LLC
Vo(t)
20 kilohms
10 microfarads
20 kilohms
100 microfarads
Vi(t)
Vo(t)
Figure P6.8 Simple Active Filter
© 1999 CRC Press LLC
CHAPTER SEVEN
TWO-PORT NETWORKS
This chapter discusses the application of MATLAB for analysis of two-port
networks. The describing equations for the various two-port network representations are given. The use of MATLAB for solving problems involving parallel, series and cascaded two-port networks is shown. Example problems involving both passive and active circuits will be solved using MATLAB.
7.1
TWO-PORT NETWORK REPRESENTATIONS
A general two-port network is shown in Figure 7.1.
I1
I2
+
+
Linear
two-port
network
V1
-
V2
-
Figure 7.1 General Two-Port Network
I 1 and V1 are input current and voltage, respectively. Also, I 2 and V2 are
output current and voltage, respectively. It is assumed that the linear two-port
circuit contains no independent sources of energy and that the circuit is initially
at rest ( no stored energy). Furthermore, any controlled sources within the linear two-port circuit cannot depend on variables that are outside the circuit.
7.1.1
z-parameters
A two-port network can be described by z-parameters as
V1 = z11 I 1 + z12 I 2
(7.1)
V2 = z 21 I 1 + z 22 I 2
(7.2)
In matrix form, the above equation can be rewritten as
© 1999 CRC Press LLC
V1 z11
V = z
2 21
z12 I 1
z22 I 2
(7.3)
The z-parameter can be found as follows
z11 =
V1
I1
I2 =0
(7.4)
z12 =
V1
I2
I1 = 0
(7.5)
z 21 =
V2
I1
I2 =0
(7.6)
z 22 =
V2
I2
I1 = 0
(7.7)
The z-parameters are also called open-circuit impedance parameters since they
are obtained as a ratio of voltage and current and the parameters are obtained
by open-circuiting port 2 ( I 2 = 0) or port1 ( I 1 = 0). The following example shows a technique for finding the z-parameters of a simple circuit.
Example 7.1
For the T-network shown in Figure 7.2, find the z-parameters.
I1
Z1
Z2
+
V1
-
Figure 7.2 T-Network
© 1999 CRC Press LLC
I2
+
Z3
V2
-
Solution
Using KVL
V1 = Z1 I 1 + Z 3 ( I 1 + I 2 ) = ( Z1 + Z 3 ) I 1 + Z 3 I 2
(7.8)
V2 = Z 2 I 2 + Z 3 ( I 1 + I 2 ) = ( Z 3 ) I 1 + ( Z 2 + Z 3 ) I 2
(7.9)
V1 Z1 + Z 3
V = Z
3
2
(7.10)
thus
I1
Z 2 + Z 3 I 2
Z3
and the z-parameters are
Z1 + Z 3
Z3
[Z] =
7.1.2
Z 2 + Z 3
Z3
(7.11)
y-parameters
A two-port network can also be represented using y-parameters. The describing equations are
I 1 = y11V1 + y12V2
(7.12)
I 2 = y 21V1 + y 22V2
(7.13)
where
V1 and V2 are independent variables and
I 1 and I 2 are dependent variables.
In matrix form, the above equations can be rewritten as
I1
I =
2
y11
y
21
y12 V1
y 22 V2
The y-parameters can be found as follows:
© 1999 CRC Press LLC
(7.14)
y11 =
I1
V1
V2 = 0
(7.15)
y12 =
I1
V2
V1 = 0
(7.16)
y 21 =
I2
V1
V2 = 0
(7.17)
y 22 =
I2
V2
V1 = 0
(7.18)
The y-parameters are also called short-circuit admittance parameters. They are
obtained as a ratio of current and voltage and the parameters are found by
short-circuiting port 2 ( V2 = 0) or port 1 ( V1 = 0). The following two examples show how to obtain the y-parameters of simple circuits.
Example 7.2
Find the y-parameters of the pi (π) network shown in Figure 7.3.
Yb
I1
I2
+
+
V1
Ya
Yc
V2
-
-
Figure 7.3 Pi-Network
Solution
Using KCL, we have
I 1 = V1Ya + (V1 − V2 )Yb = V1 (Ya + Yb ) − V2 Yb
© 1999 CRC Press LLC
(7.19)
I 2 = V2 Yc + (V2 − V1 )Yb = −V1Yb + V2 (Yb + Yc )
(7.20)
Comparing Equations (7.19) and (7.20) to Equations (7.12) and (7.13), the yparameters are
Ya + Yb
− Yb
[Y ] =
− Yb
Yb + Yc
(7.21)
Example 7.3
Figure 7.4 shows the simplified model of a field effect transistor. Find its yparameters.
I1
I2
+
C3
C1
V1
+
gmV1
Y2
-
V2
-
Figure 7.4 Simplified Model of a Field Effect Transistor
Using KCL,
I 1 = V1 sC1 + (V1 − V2 ) sC3 = V1 ( sC1 + sC3 ) + V2 ( − sC3 )
(7.22)
I 2 = V2 Y2 + g mV1 + (V2 − V1 ) sC3 = V1 ( g m − sC3 ) + V2 (Y2 + sC3 )
(7.23)
Comparing the above two equations to Equations (7.12) and (7.13), the yparameters are
© 1999 CRC Press LLC
sC1 + sC3
g m − sC3
[Y ] =
7.1.3
− sC3
Y2 + sC3
(7.24)
h-parameters
A two-port network can be represented using the h-parameters. The describing
equations for the h-parameters are
V1 = h11 I 1 + h12V2
(7.25)
I 2 = h21 I 1 + h22V2
(7.26)
where
I 1 and V2 are independent variables and
V1 and I 2 are dependent variables.
In matrix form, the above two equations become
V1
I =
2
h11
h
21
h12 I 1
h22 V2
(7.27)
The h-parameters can be found as follows:
© 1999 CRC Press LLC
h11 =
V1
I1
V2 = 0
(7.28)
h12 =
V1
V2
I1 = 0
(7.29)
h21 =
I2
I1
V2 = 0
(7.30)
h22 =
I2
V2
I1 = 0
(7.31)
The h-parameters are also called hybrid parameters since they contain both
open-circuit parameters ( I 1 = 0 ) and short-circuit parameters ( V2 = 0 ). The
h-parameters of a bipolar junction transistor are determined in the following
example.
Example 7.4
A simplified equivalent circuit of a bipolar junction transistor is shown in Figure 7.5, find its h-parameters.
Z1
I1
I2
+
+
β I1
V1
Y2
V2
-
-
Figure 7.5 Simplified Equivalent Circuit of a Bipolar Junction
Transistor
Solution
Using KCL for port 1,
V1 = I 1 Z1
(7.32)
Using KCL at port 2, we get
I 2 = βI 1 + Y2V2
(7.33)
Comparing the above two equations to Equations (7.25) and (7.26) we get the
h-parameters.
Z1 0
β Y2
[ h] =
© 1999 CRC Press LLC
`
(7.34)
7.1.4
Transmission parameters
A two-port network can be described by transmission parameters.
scribing equations are
The de-
V1 = a11V2 − a12 I 2
(7.35)
I 1 = a 21V2 − a 22 I 2
(7.36)
where
V2 and I 2 are independent variables and
V1 and I 1 are dependent variables.
In matrix form, the above two equations can be rewritten as
V1
I =
1
a12 V2
a 22 − I 2
a11
a
21
(7.37)
The transmission parameters can be found as
a11 =
V1
V2
a12 = −
a 21 =
V1
I2
I1
V2
a 22 = −
I2 =0
V2 = 0
I2 =0
I1
I2
V2 = 0
(7.38)
(7.39)
(7.40)
(7.41)
The transmission parameters express the primary (sending end) variables
V1
I 1 in terms of the secondary (receiving end) variables V2 and - I 2 . The
negative of I 2 is used to allow the current to enter the load at the receiving
and
end. Examples 7.5 and 7.6 show some techniques for obtaining the transmission parameters of impedance and admittance networks.
© 1999 CRC Press LLC
Example 7.5
Find the transmission parameters of Figure 7.6.
Z1
I1
I2
+
+
V1
V2
-
-
Figure 7.6 Simple Impedance Network
Solution
By inspection,
I1 = − I 2
(7.42)
Using KVL,
V1 = V2 + Z1 I 1
Since
(7.43)
I 1 = − I 2 , Equation (7.43) becomes
V1 = V2 − Z1 I 2
(7.44)
Comparing Equations (7.42) and (7.44) to Equations (7.35) and (7.36), we
have
a11 = 1
a 21 = 0
© 1999 CRC Press LLC
a12 = Z1
a 22 = 1
(7.45)
Example 7.6
Find the transmission parameters for the network shown in Figure 7.7.
I2
I1
+
+
V1
V2
Y2
-
-
Figure 7.7 Simple Admittance Network
Solution
By inspection,
V1 = V2
(7.46)
Using KCL, we have
I 1 = V2 Y2 − I 2
(7.47)
Comparing Equations (7.46) and 7.47) to equations (7.35) and (7.36) we have
a11 = 1
a 21 = Y2
a12 = 0
a 22 = 1
(7.48)
Using the describing equations, the equivalent circuits of the various two-port
network representations can be drawn. These are shown in Figure 7.8.
Z22
Z11
I2
I1
+
+
V1
Z12 I1
Z21 I1
-
-
(a)
© 1999 CRC Press LLC
V2
I1
I2
+
+
V1
Y11 V1
Y12 V2
Y21 V1
Y22 V2
V2
-
-
(b)
h11
I1
I2
+
+
h12 V2
V1
h21 I1
h22
-
V2
-
(c )
Figure 7.8 Equivalent Circuit of Two-port Networks (a) zparameters, (b) y-parameters and (c ) h-parameters
7.2
INTERCONNECTION OF TWO-PORT NETWORKS
Two-port networks can be connected in series, parallel or cascade. Figure 7.9
shows the various two-port interconnections.
I2
I1
+
V1'
+
-
+
[Z]1
-
V2'
V1
+
V2
-
V1''
+
-
+
[Z]2
-
V2''
(a) Series-connected Two-port Network
© 1999 CRC Press LLC
-
I1
I1'
I2'
I2
+
V1
-
+
V2
-
[Y] 1
I1''
I2''
[Y] 2
(b) Parallel-connected Two-port Network
Ix
I1
+
I2
+
V1
+
Vx
[A]1
-
[A]2
-
V2
-
(c ) Cascade Connection of Two-port Network
Figure 7.9 Interconnection of Two-port Networks (a) Series
(b) Parallel (c ) Cascade
It
can
be
shown
that
[ Z ]1 , [ Z ] 2 , [ Z ] 3 , ..., [ Z ] n
if two-port networks with z-parameters
are connected in series, then the equivalent two-
port z-parameters are given as
[ Z ] eq = [ Z ]1 + [ Z ] 2 + [ Z ] 3 + ...+[ Z ] n
If two-port networks with y-parameters
[Y ]1 ,[Y ] 2 , [Y ] 3 , ..., [Y ] n
(7.49)
are con-
nected in parallel, then the equivalent two-port y-parameters are given as
[Y ] eq = [Y ]1 + [Y ] 2 + [Y ] 3 + ...+[Y ] n
(7.50)
When several two-port networks are connected in cascade, and the individual
networks have transmission parameters [ A]1 , [ A] 2 , [ A] 3 , ..., [ A] n , then the
equivalent two-port parameter will have a transmission parameter given as
[ A] eq = [ A]1 * [ A] 2 * [ A] 3 * ...*[ A] n
© 1999 CRC Press LLC
(7.51)
The following three examples illustrate the use of MATLAB for determining
the equivalent parameters of interconnected two-port networks.
Example 7.7
Find the equivalent y-parameters for the bridge T-network shown in Figure
7.10.
Z4
Z1
I1
Z2
I2
+
+
V1
Z3
V2
-
-
Figure 7.10 Bridge-T Network
Solution
The bridge-T network can be redrawn as
N1
Z4
I1
+
V1
Z1
I2
Z2
+
Z3
V2
_
N2
Figure 7.11 An Alternative Representation of Bridge-T Network
© 1999 CRC Press LLC
From Example 7.1, the z-parameters of network N2 are
Z1 + Z3
Z3
[Z ] =
Z2 + Z3
Z3
We can convert the z-parameters to y-parameters [refs. 4 and 6] and we get
Z 2 + Z3
Z1Z2 + Z1Z3 + Z 2 Z3
− Z3
y12 =
Z1 Z2 + Z1Z3 + Z2 Z3
− Z3
y 21 =
Z1Z2 + Z1Z3 + Z 2 Z3
Z1 + Z3
y 22 = −
Z1 Z2 + Z1 Z3 + Z2 Z3
y11 =
(7.52)
From Example 7.5, the transmission parameters of network N1 are
a11 = 1
a12 = Z4
a21 = 0
a 22 = 1
We convert the transmission parameters to y-parameters[ refs. 4 and 6] and we
get
y11 =
1
Z4
y12 = −
1
Z4
1
y 21 = −
Z4
y 22 =
© 1999 CRC Press LLC
1
Z4
(7.53)
Using Equation (7.50), the equivalent y-parameters of the bridge-T network
are
1
Z 2 + Z3
+
Z4 Z1 Z2 + Z1 Z3 + Z2 Z3
Z3
1
=−
−
Z4 Z1 Z2 + Z1Z3 + Z 2 Z3
1
Z3
=−
−
Z4 Z1Z2 + Z1Z3 + Z2 Z3
1
Z1 + Z3
=
+
Z4 Z1Z 2 + Z1Z3 + Z2 Z3
y11eq =
y12 eq
y 21eq
y 22 eq
(7.54)
Example 7.8
Find the transmission parameters of Figure 7.12.
Z1
Y2
Figure 7.12 Simple Cascaded Network
© 1999 CRC Press LLC
Solution
Figure 7.12 can be redrawn as
Z1
Y2
N1
N2
Figure 7.13 Cascade of Two Networks N1 and N2
From Example 7.5, the transmission parameters of network N1 are
a11 = 1
a 21 = 0
a12 = Z1
a 22 = 1
From Example 7.6, the transmission parameters of network N2 are
a11 = 1
a 21 = Y2
a12 = 0
a 22 = 1
From Equation (7.51), the transmission parameters of Figure 7.13 are
a11
a
21
© 1999 CRC Press LLC
a12
1 Z 1 1
=
a 22 eq 0 1 Y2
0 1 + Z1Y2
=
1 Y2
Z1
1
(7.55)
Example 7.9
Find the transmission parameters for the cascaded system shown in Figure
7.14. The resistance values are in Ohms.
I1
2
4
8
16
I2
+
+
V1
1
2
4
8
V2
_
N1
N2
N3
N4
Figure 7.14 Cascaded Resistive Network
Solution
Figure 7.14 can be considered as four networks, N1, N2, N3, and N4 connected in cascade. From Example 7.8, the transmission parameters of Figure
7.12 are
3 2
1 1
[a ] N 1 =
3 4
0.5 1
[a ] N 2 =
3
8
[a ] N 3 =
0.25 1
3
16
[a ] N 4 =
.
1
0125
The transmission parameters of Figure 7.14 can be obtained using the following MATLAB program.
© 1999 CRC Press LLC
MATLAB Script
diary ex7_9.dat
% Transmission parameters of cascaded network
a1 = [3 2; 1 1];
a2 = [3 4; 0.5 1];
a3 = [3 8; 0.25 1];
a4 = [3 16; 0.125 1];
% equivalent transmission parameters
a = a1*(a2*(a3*a4))
diary
The value of matrix a is
a=
112.2500
39.3750
7.3
630.0000
221.0000
TERMINATED TWO-PORT NETWORKS
In normal applications, two-port networks are usually terminated. A terminated two-port network is shown in Figure 7.4.
Zin
Zg
I1
I2
+
+
Vg
V1
-
V2
ZL
-
Figure 7.15 Terminated Two-Port Network
Vg and Z g are the source generator voltage and impedance, respectively. Z L is the load impedance. If we use z-parameter repre-
In the Figure 7.15,
sentation for the two-port network, the voltage transfer function can be shown
to be
© 1999 CRC Press LLC
V2
z 21 Z L
=
Vg ( z11 + Z g )( z 22 + Z L ) − z12 z 21
(7.56)
and the input impedance,
Zin = z11 −
z12 z 21
z 22 + Z L
(7.57)
and the current transfer function,
I2
z 21
=−
I1
z 22 + Z L
(7.58)
A terminated two-port network, represented using the y-parameters, is shown
in Figure 7.16.
Yin
I1
I2
+
Ig
Yg
Figure 7.16
+
+
Vg
V1
-
-
V2
ZL
-
A Terminated Two-Port Network with y-parameters
Representation
It can be shown that the input admittance,
Yin = y11 −
[Y]
Yin , is
y12 y 21
y 22 + YL
(7.59)
and the current transfer function is given as
I2
y 21YL
=
I g ( y11 + Yg )( y 22 + YL ) − y12 y 21
© 1999 CRC Press LLC
(7.60)
and the voltage transfer function
V2
y21
=−
Vg
y 22 + YL
(7.61)
A doubly terminated two-port network, represented by transmission parameters, is shown in Figure 7.17.
Zin
Zg
I1
I2
+
+
Vg
V1
-
[A]
V2
ZL
-
Figure 7.17 A Terminated Two-Port Network with Transmission
Parameters Representation
The voltage transfer function and the input impedance of the transmission parameters can be obtained as follows. From the transmission parameters, we
have
V1 = a11V2 − a12 I 2
(7.62)
I 1 = a 21V2 − a 22 I 2
(7.63)
From Figure 7.6,
V2 = − I 2 Z L
(7.64)
Substituting Equation (7.64) into Equations (7.62) and (7.63), we get the input
impedance,
Zin =
© 1999 CRC Press LLC
a11 Z L + a12
a 21 Z L + a 22
(7.65)
From Figure 7.17, we have
V1 = V g − I 1 Z g
(7.66)
Substituting Equations (7.64) and (7.66) into Equations (7.62) and (7.63), we
have
Vg − I 1 Z g = V2 [a11 +
I 1 = V2 [a 21 +
a12
]
ZL
a 22
]
ZL
(7.67)
(7.68)
Substituting Equation (7.68) into Equation (7.67), we get
Vg − V2 Z g [a 21 +
a 22
a12
] = V2 [a11 +
]
ZL
ZL
(7.69)
Simplifying Equation (7.69), we get the voltage transfer function
V2
ZL
=
Vg (a11 + a 21 Z g ) Z L + a12 + a 22 Z g
(7.70)
The following examples illustrate the use of MATLAB for solving terminated
two-port network problems.
Example 7.10
Assuming that the operational amplifier of Figure 7.18 is ideal,
(a)
Find the z-parameters of Figure 7.18.
(b)
If the network is connected by a voltage source with source
resistance of 50Ω and a load resistance of 1 KΩ, find the voltage
gain.
(c )
Use MATLAB to plot the magnitude response.
© 1999 CRC Press LLC
10 kilohms
I3
I1
R3
2 kilohms
2 kilohms
R2
I2
1 kilohms
R4
+
+
R1
V1
C = 0.1 microfarads
1
___
sC
-
V2
-
Figure 7.18 An Active Lowpass Filter
Solution
Using KVL,
V1 = R1 I 1 +
I1
sC
(7.71)
V2 = R4 I 2 + R3 I 3 + R2 I 3
(7.72)
From the concept of virtual circuit discussed in Chapter 11,
R2 I 3 =
I1
sC
(7.73)
Substituting Equation (7.73) into Equation (7.72), we get
V2 =
(R
2
+ R3 )I 1
sCR2
+ R4 I 2
(7.74)
Comparing Equations (7.71) and (7.74) to Equations (7.1) and (7.2), we have
© 1999 CRC Press LLC
z11 = R1 +
1
sC
z12 = 0
R3 1
z 21 = 1 +
R2 sC
(7.75)
z 22 = R4
From Equation (7.56), we get the voltage gain for a terminated two-port network. It is repeated here.
V2
z 21 Z L
=
Vg ( z11 + Z g )( z 22 + Z L ) − z12 z 21
Substituting Equation (7.75) into Equation (7.56), we have
R3
)Z
V2
R2 L
=
Vg ( R4 + Z L )[1 + sC ( R1 + Z g )]
(1 +
(7.76)
Z g = 50 Ω , Z L = 1 KΩ, R3 = 10 KΩ, R2 = 1 KΩ, R4 = 2 KΩ
. µF , Equation (7.76) becomes
and C = 01
For
V2
2
=
V g [1 + 105
. * 10 − 4 s]
The MATLAB script is
%
num = [2];
den = [1.05e-4 1];
w = logspace(1,5);
h = freqs(num,den,w);
f = w/(2*pi);
mag = 20*log10(abs(h)); % magnitude in dB
semilogx(f,mag)
title('Lowpass Filter Response')
xlabel('Frequency, Hz')
© 1999 CRC Press LLC
(7.77)
ylabel('Gain in dB')
The frequency response is shown in Figure 7.19.
Figure 7.19 Magnitude Response of an Active Lowpass Filter
SELECTED BIBLIOGRAPHY
1.
MathWorks, Inc., MATLAB, High-Performance Numeric
Computation Software, 1995.
2.
Biran, A. and Breiner, M., MATLAB for Engineers, AddisonWesley, 1995.
3.
Etter, D.M., Engineering Problem Solving with MATLAB, 2nd Edition, Prentice Hall, 1997.
© 1999 CRC Press LLC
4.
Nilsson, J.W., Electric Circuits, 3rd Edition, Addison-Wesley
Publishing Company, 1990.
5.
Meader, D.A., Laplace Circuit Analysis and Active Filters,
Prentice Hall, 1991.
6.
Johnson, D. E. Johnson, J.R., and Hilburn, J.L. Electric Circuit
Analysis, 3rd Edition, Prentice Hall, 1997.
7.
Vlach, J.O., Network Theory and CAD, IEEE Trans. on Education,
Vol. 36, No. 1, Feb. 1993, pp. 23 - 27.
EXERCISES
(a) Find the transmission parameters of the circuit shown in Figure
P7.1a. The resistance values are in ohms.
7.1
1
2
4
Figure P7.1a Resistive T-Network
(b) From the result of part (a), use MATLAB to find the transmission
parameters of Figure P7.2b. The resistance values are in ohms.
1
2
4
2
4
8
8
4
16
Figure P7.1b Cascaded Resistive Network
7.2
© 1999 CRC Press LLC
32
8
Find the y-parameters of the circuit shown in Figure P7.2
The resistance values are in ohms.
32
20
I1
2
2
I2
+
+
V1
4
10
10
V2
-
-
Figure P7.2 A Resistive Network
7.3
(a)
Show that for the symmetrical lattice structure shown in
Figure P7.3,
z11 = z 22 = 0.5( Z c + Z d )
z12 = z 21 = 0.5( Z c − Z d )
(b)
If Z c = 10 Ω,
parameters.
Z d = 4 Ω, find the equivalent yZd
ZC
ZC
Zd
Figure P7.3 Symmetrical Lattice Structure
© 1999 CRC Press LLC
7.4
(a) Find the equivalent z-parameters of Figure P7.4.
(b) If the network is terminated by a load of 20 ohms and connected
to a source of VS with a source resistance of 4 ohms, use MATLAB
to plot the frequency response of the circuit.
2H
2H
+
+
10 Ohms
0.25 F
-
5 Ohms
5 Ohms
Figure P7.4 Circuit for Problem 7.4
7.5
For Figure P7.5
(a) Find the transmission parameters of the RC ladder network.
(b) Obtain the expression for
(c)
V2
.
V1
Use MATLAB to plot the phase characteristics of
R
R
R
+
V1
+
C
C
-
C
V2
-
Figure P7.5 RC Ladder Network
© 1999 CRC Press LLC
V2
.
V1
For the circuit shown in Figure P7.6,
(a) Find the y-parameters.
(b) Find the expression for the input admittance.
(c) Use MATLAB to plot the input admittance as a function of
frequency.
7.6
R3
C
I2
I2
+
V1
+
L
R1
L
R2
V2
-
-
Figure P7.6 Circuit for Problem 7.6
7.7
For the op amp circuit shown in Figure P7.7, find the y-parameters.
R3
I1
+
R5
+
R4
V1
V2
R1
R2
-
-
Figure P7.7 Op Amp Circuit
© 1999 CRC Press LLC
I2
CHAPTER EIGHT
FOURIER ANALYSIS
In this chapter, Fourier analysis will be discussed. Topics covered are Fourier series expansion, Fourier transform, discrete Fourier transform, and fast
Fourier transform. Some applications of Fourier analysis, using MATLAB,
will also be discussed.
8.1
If a function
FOURIER SERIES
g (t ) is periodic with period Tp , i.e.,
g (t ) = g (t ± Tp )
(8.1)
and in any finite interval g ( t ) has at most a finite number of discontinuities
and a finite number of maxima and minima (Dirichlets conditions), and in
addition,
Tp
∫ g(t )dt < ∞
(8.2)
0
then
g (t ) can be expressed with series of sinusoids. That is,
∞
a0
+ ∑ a n cos(nw0 t ) + bn sin(nw0 t )
g (t ) =
2 n =1
where
w0 =
2π
Tp
(8.4)
and the Fourier coefficients
tions.
2
an =
Tp
© 1999 CRC Press LLC
(8.3)
a n and bn are determined by the following equa-
t o + Tp
∫ g(t ) cos(nw t )dt
0
to
n = 0, 1,2, …
(8.5)
2
bn =
Tp
t o + Tp
∫ g (t ) sin(nw t )dt
n = 0, 1, 2 …
0
(8.6)
to
Equation (8.3) is called the trigonometric Fourier series. The term
a0
in
2
Equation (8.3) is the dc component of the series and is the average value of
g (t ) over a period. The term a n cos(nw0 t ) + bn sin(nw0 t ) is called the nth harmonic. The first harmonic is obtained when n = 1. The latter is also
called the fundamental with the fundamental frequency of ωo . When n = 2,
we have the second harmonic and so on.
Equation (8.3) can be rewritten as
∞
a0
+ ∑ A cos(nw0 t + Θ n )
g (t ) =
2 n =1 n
(8.7)
An = a n2 + bn2
(8.8)
b
Θ n = − tan −1 n
an
(8.9)
where
and
The total power in
1
P=
Tp
g (t ) is given by the Parseval’s equation:
t o + Tp
∫g
∞
2
to
(t )dt = A + ∑
2
dc
n =1
An2
2
(8.10)
where
a0
A =
2
2
dc
2
(8.11)
The following example shows the synthesis of a square wave using Fourier
series expansion.
© 1999 CRC Press LLC
Example 8.1
Using Fourier series expansion, a square wave with a period of 2 ms, peak-topeak value of 2 volts and average value of zero volt can be expressed as
g (t ) =
4 ∞
1
sin[(2n − 1)2πf 0 t ]
∑
π n =1 (2n − 1)
(8.12)
where
f 0 = 500 Hz
if
a (t ) is given as
4 12
1
sin[(2n − 1)2πf 0 t ]
a (t ) = ∑
π n =1 (2n − 1)
Write a MATLAB program to plot
ms and to show that
a (t ) from 0 to 4 ms at intervals of 0.05
a (t ) is a good approximation of g(t).
Solution
MATLAB Script
% fourier series expansion
f = 500; c = 4/pi; dt = 5.0e-05;
tpts = (4.0e-3/5.0e-5) + 1;
for n = 1: 12
for m = 1: tpts
s1(n,m) = (4/pi)*(1/(2*n - 1))*sin((2*n - 1)*2*pi*f*dt*(m-1));
end
end
for m = 1:tpts
a1 = s1(:,m);
a2(m) = sum(a1);
end
f1 = a2';
t = 0.0:5.0e-5:4.0e-3;
clg
plot(t,f1)
xlabel('Time, s')
© 1999 CRC Press LLC
(8.13)
ylabel('Amplitude, V')
title('Fourier series expansion')
Figure 8.1 shows the plot of
a (t ) .
Figure 8.1 Approximation to Square Wave
By using the Euler’s identity, the cosine and sine functions of Equation (8.3)
can be replaced by exponential equivalents, yielding the expression
∞
g (t ) =
∑c
n =−∞
n
exp( jnw0 t )
(8.14)
where
1
cn =
Tp
and
w0 =
© 1999 CRC Press LLC
Tp / 2
∫ g (t ) exp(− jnw t )dt
0
−t p / 2
2π
Tp
(8.15)
Equation (8.14) is termed the exponential Fourier series expansion. The coefficient cn is related to the coefficients a n and bn of Equations (8.5) and (8.6)
by the expression
cn =
In addition,
tion
(8.16)
cn relates to An and φn of Equations (8.8) and (8.9) by the rela-
cn =
The plot of
b
1 2
a n + bn2 ∠ − tan −1 ( n )
2
an
An
∠Θ n
2
(8.17)
cn versus frequency is termed the discrete amplitude spectrum or
the line spectrum. It provides information on the amplitude spectral components of g ( t ). A similar plot of ∠cn versus frequency is called the discrete phase spectrum and the latter gives information on the phase components
with respect to the frequency of g ( t ) .
If an input signal
x n (t )
x n (t ) = cn exp( jnwo t )
passes through a system with transfer function
system
(8.18)
H ( w) , then the output of the
y n (t ) is
y n (t ) = H ( jnwo )cn exp( jnwo t )
(8.19)
The block diagram of the input/output relation is shown in Figure 8.2.
xn(t)
H(s)
yn(t)
Figure 8.2 Input/Output Relationship
However, with an input
excitations,
© 1999 CRC Press LLC
x (t ) consisting of a linear combination of complex
x n (t ) =
∞
∑
n =−∞
cn exp( jnwo t )
(8.20)
the response at the output of the system is
y n (t ) =
∞
∑
n =−∞
H ( jnwo )cn exp( jnwo t )
(8.21)
The following two examples show how to use MATLAB to obtain the coefficients of Fourier series expansion.
Example 8.2
For the full-wave rectifier waveform shown in Figure 8.3, the period is 0.0333s
and the amplitude is 169.71 Volts.
(a)
Write a MATLAB program to obtain the exponential Fourier series
coefficients cn for n = 0,1, 2, .. , 19
(b)
Find the dc value.
(c)
Plot the amplitude and phase spectrum.
Figure 8.3 Full-wave Rectifier Waveform
© 1999 CRC Press LLC
Solution
diary ex8_2.dat
% generate the full-wave rectifier waveform
f1 = 60;
inv = 1/f1; inc = 1/(80*f1); tnum = 3*inv;
t = 0:inc:tnum;
g1 = 120*sqrt(2)*sin(2*pi*f1*t);
g = abs(g1);
N = length(g);
%
% obtain the exponential Fourier series coefficients
num = 20;
for i = 1:num
for m = 1:N
cint(m) = exp(-j*2*pi*(i-1)*m/N)*g(m);
end
c(i) = sum(cint)/N;
end
cmag = abs(c);
cphase = angle(c);
%print dc value
disp('dc value of g(t)'); cmag(1)
% plot the magnitude and phase spectrum
f = (0:num-1)*60;
subplot(121), stem(f(1:5),cmag(1:5))
title('Amplitude spectrum')
xlabel('Frequency, Hz')
subplot(122), stem(f(1:5),cphase(1:5))
title('Phase spectrum')
xlabel('Frequency, Hz')
diary
dc value of g(t)
ans =
107.5344
Figure 8.4 shows the magnitude and phase spectra of Figure 8.3.
© 1999 CRC Press LLC
Figure 8.4 Magnitude and Phase Spectra of a Full-wave
Rectification Waveform
Example 8.3
The periodic signal shown in Figure 8.5 can be expressed as
−1≤ t < 1
g (t ) = e −2 t
g (t + 2) = g ( t )
(i) Show that its exponential Fourier series expansion can be expressed as
g (t ) =
( −1) n (e 2 − e −2 )
∑ 2(2 + jnπ ) exp( jnπt )
n =−∞
∞
(ii) Using a MATLAB program, synthesize
© 1999 CRC Press LLC
g (t ) using 20 terms, i.e.,
(8.22)
( −1) n (e 2 − e −2 )
∑ 2(2 + jnπ ) exp( jnπt )
n =−10
10
∧
g( t ) =
g(t)
1
0
2
Figure 8.5 Periodic Exponential Signal
Solution
(i)
∞
g (t ) =
∑c
n =−∞
n
exp( jnwo t )
where
Tp / 2
1
cn =
Tp
and
wo =
∫ g (t ) exp( − jnw t )dt
o
− Tp / 2
2π 2π
=
=π
2
Tp
1
1
cn = ∫ exp( −2t ) exp( − jnπt )dt
2 −1
cn =
thus
© 1999 CRC Press LLC
( −1) n (e 2 − e −2 )
2(2 + jnπ )
4
t(s)
g (t ) =
( −1) n (e 2 − e −2 )
∑ 2(2 + jnπ ) exp( jnπt )
n =−∞
∞
(ii) MATLAB Script
% synthesis of g(t) using exponential Fourier series expansion
dt = 0.05;
tpts = 8.0/dt +1;
cst = exp(2) - exp(-2);
for n = -10:10
for m = 1:tpts
g1(n+11,m) = ((0.5*cst*((-1)^n))/(2+j*n*pi))*(exp(j*n*pi*dt*(m1)));
end
end
for m = 1: tpts
g2 = g1(:,m);
g3(m) = sum(g2);
end
g = g3';
t = -4:0.05:4.0;
plot(t,g)
xlabel('Time, s')
ylabel('Amplitude')
title('Approximation of g(t)')
Figure 8.6 shows the approximation of
© 1999 CRC Press LLC
g (t ) .
Figure 8.6 An Approximation of
8.2
g (t ) .
FOURIER TRANSFORMS
g (t ) is a nonperiodic deterministic signal expressed as a function of time
t, then the Fourier transform of g ( t ) is given by the integral expression:
If
∞
G( f ) =
∫ g (t ) exp(− j 2πft )dt
(8.23)
−∞
where
j = −1
and
f denotes frequency
g (t ) can be obtained from the Fourier transform G ( f ) by the Inverse Fourier Transform formula:
© 1999 CRC Press LLC
∞
g (t ) =
∫ G( f ) exp( j 2πft )df
(8.24)
−∞
g (t ) to be Fourier transformable, it should satisfy the Dirichlet’s
conditions that were discussed in Section 8.1. If g ( t ) is continuous and nonperiodic, then G ( f ) will be continuous and periodic. However, if g(t) is
continuous and periodic, then G ( f ) will discrete and nonperiodic; that is
For a signal
g (t ) = g (t ± nTp )
(8.25)
where
Tp = period
then the Fourier transform of
1
G( f ) =
Tp
∞
g (t ) is
∑c δ( f
n =−∞
n
−
1
)
Tp
(8.26)
where
1
cn =
Tp
8.2.1
If
Tp / 2
∫ g (t ) exp( − j 2πnf
o
t )dt
(8.27)
−t p / 2
Properties of Fourier transform
g (t ) and G ( f ) are Fourier transform pairs, and they are expressed as
g (t ) ⇔ G ( f )
(8.28)
then the Fourier transform will have the following properties:
Linearity
ag1 (t ) + bg 2 (t ) ⇔ aG1 ( f ) + bG2 ( f )
where
a and b are constants
© 1999 CRC Press LLC
(8.29)
Time scaling
1 f
G
a a
g (at ) ⇔
(8.30)
Duality
G (t ) ⇔ g ( − f )
(8.31)
Time shifting
g (t − t 0 ) ⇔ G ( f ) exp( − j 2πft 0 )
(8.32)
Frequency Shifting
exp( j 2 f C t ) g (t ) ⇔ G ( f − f C )
(8.33)
Definition in the time domain
dg (t )
⇔ j 2πfG ( f )
dt
(8.34)
Integration in the time domain
t
∫ g (τ )dτ ⇔
−∞
1
G (0)
δ ( f ) δ (f)
G( f ) +
2
j 2πf
(8.35)
Multiplication in the time domain
∞
∫ G ( λ )G
( f − λ ) dλ
(8.36)
(t − τ )dτ ⇔ G1 ( f )G2 ( f )
(8.37)
g1 ( t ) g 2 ( t ) ⇔
1
2
−∞
Convolution in the time domain
∞
∫ g (τ ) g
1
−∞
© 1999 CRC Press LLC
2
8.3
DISCRETE AND FAST FOURIER TRANSFORMS
Fourier series links a continuous time signal into the discrete-frequency domain. The periodicity of the time-domain signal forces the spectrum to be discrete. The discrete Fourier transform of a discrete-time signal g[ n] is given
as
G[ k ] =
N −1
∑ g[n]exp( − j 2πnk / N )
k = 0,1, …, N-1
(8.38)
n = 0,1,…, N-1
(8.39)
n=0
The inverse discrete Fourier transform,
g[n] =
g[n] is
N −1
∑ G[ k ]exp( j 2πnk / N )
k =0
where
g[n] . It is also
the total number frequency sequence values in G[ k ] .
N
is the number of time sequence values of
T
is the time interval between two consecutive samples of the
input sequence g[ n] .
F
is the frequency interval between two consecutive samples
of the output sequence G[ k ] .
N, T, and F are related by the expression
NT =
1
F
(8.40)
NT is also equal to the record length. The time interval, T, between samples
should be chosen such that the Shannon’s Sampling theorem is satisfied. This
means that T should be less than the reciprocal of 2 f H , where f H is the
highest significant frequency component in the continuous time signal
from which the sequence g[ n] was obtained.
require N to be an integer power of 2.
g (t )
Several fast DFT algorithms
A discrete-time function will have a periodic spectrum. In DFT, both the time
function and frequency functions are periodic. Because of the periodicity of
DFT, it is common to regard points from n = 1 through n = N/2 as positive,
© 1999 CRC Press LLC
and points from n = N/2 through n = N - 1 as negative frequencies. In addition, since both the time and frequency sequences are periodic, DFT values at
points n = N/2 through n = N - 1 are equal to the DFT values at points n = N/2
through n = 1.
In general, if the time-sequence is real-valued, then the DFT will have real
components which are even and imaginary components that are odd. Similarly, for an imaginary valued time sequence, the DFT values will have an odd
real component and an even imaginary component.
If we define the weighting function
WN = e
− j 2π
N
WN as
= e − j 2 πFT
(8.41)
Equations (8.38) and (8.39) can be re-expressed as
G[ k ] =
N −1
∑ g[n]W
n=0
kn
N
(8.42)
and
g[n] =
N −1
∑ G[ k ]W
k =0
− kn
N
(8.43)
The Fast Fourier Transform, FFT, is an efficient method for computing the
discrete Fourier transform. FFT reduces the number of computations needed
for computing DFT. For example, if a sequence has N points, and N is an in-
N 2 operations, whereas FFT requires
N
N
log 2 ( N ) complex multiplication,
log 2 ( N ) complex additions and
2
2
N
log 2 ( N ) subtractions. For N = 1024, the computational reduction from
2
tegral power of 2, then DFT requires
DFT to FFT is more than 200 to 1.
The FFT can be used to (a) obtain the power spectrum of a signal, (b) do digital filtering, and (c) obtain the correlation between two signals.
© 1999 CRC Press LLC
8.3.1
MATLAB function fft
The MATLAB function for performing Fast Fourier Transforms is
fft ( x )
where
x is the vector to be transformed.
fft ( x , N )
is also MATLAB command that can be used to obtain N-point fft. The vector
x is truncated or zeros are added to N, if necessary.
The MATLAB functions for performing inverse fft is
ifft ( x ).
[z , z ] = fftplot ( x, ts)
m
p
is used to obtain fft and plot the magnitude
zm and z p of DFT of x. The
sampling interval is ts. Its default value is 1. The spectra are plotted versus
the digital frequency F. The following three examples illustrate usage of
MATLAB function fft.
Example 8.4
x[n] = ( 1, 2, 1). (a) Calculate the DFT of x[n] . (b)
Use the fft algorithm to find DFT of x[ n] . (c) Compare the results of (a)
Given the sequence
and (b).
Solution
(a) From Equation (8.42)
G[ k ] =
N −1
∑ g[n]W
n=0
From Equation (8.41)
© 1999 CRC Press LLC
kn
N
W30 = 1
W31 = e
−
j 2π
3
−
j 4π
3
W32 = e
= −0.5 − j 0.866
= −0.5 + j 0.866
W33 = W30 = 1
W34 = W31
Using Equation (8.41), we have
2
G[0] = ∑ g[n]W30 = 1 + 2 + 1 = 4
n =0
2
G[1] = ∑ g[n]W3n = g[0]W30 + g[1]W31 + g[2]W32
n=0
. − j0866
. ) + (−05
. + j0866
. ) = −05
. − j0866
.
= 1 + 2(−05
2
G[2] = ∑ g[n]W32n = g[0]W30 + g[1]W32 + g[2]W34
n =0
. + j0866
. ) + (−05
. − j0866
. ) = −05
. + j 0866
.
= 1 + 2(−05
(b) The MATLAB program for performing the DFT of
x[n] is
MATLAB Script
diary ex8_4.dat
%
x = [1 2 1];
xfft = fft(x)
diary
The results are
xfft =
4.0000
-0.5000 - 0.8660i
-0.5000 + 0.8660i
(c) It can be seen that the answers obtained from parts (a) and (b) are
identical.
© 1999 CRC Press LLC
Example 8.5
Signal
g (t ) is given as
g (t ) = 4e −2 t cos[2π (10)t ]u(t )
g (t ) , i.e., G ( f ) .
(a)
Find the Fourier transform of
(b)
Find the DFT of g ( t ) when the sampling interval is 0.05 s with N
= 1000.
Find the DFT of g ( t ) when the sampling interval is 0.2 s with N =
250.
Compare the results obtained from parts a, b, and c.
(c)
(d)
Solution
(a)
g (t ) can be expressed as
1
1
g (t ) = 4e − 2 t e j 20πt + e − j 20πt u(t )
2
2
Using the frequency shifting property of the Fourier Transform, we get
G( f ) =
2
2
+
2 + j 2π ( f − 10) 2 + j 2π ( f + 10)
(b, c) The MATLAB program for computing the DFT of
MATLAB Script
% DFT of g(t)
% Sample 1, Sampling interval of 0.05 s
ts1 = 0.05; % sampling interval
fs1 = 1/ts1; % Sampling frequency
n1 = 1000; % Total Samples
m1 = 1:n1; % Number of bins
sint1 = ts1*(m1 - 1); % Sampling instants
freq1 = (m1 - 1)*fs1/n1; % frequencies
gb = (4*exp(-2*sint1)).*cos(2*pi*10*sint1);
gb_abs = abs(fft(gb));
subplot(121)
© 1999 CRC Press LLC
g (t ) is
plot(freq1, gb_abs)
title('DFT of g(t), 0.05s Sampling interval')
xlabel('Frequency (Hz)')
% Sample 2, Sampling interval of 0.2 s
ts2 = 0.2; % sampling interval
fs2 = 1/ts2; % Sampling frequency
n2 = 250; % Total Samples
m2 = 1:n2; % Number of bins
sint2 = ts2*(m2 - 1); % Sampling instants
freq2 = (m2 - 1)*fs2/n2; % frequencies
gc = (4*exp(-2*sint2)).*cos(2*pi*10*sint2);
gc_abs = abs(fft(gc));
subplot(122)
plot(freq2, gc_abs)
title('DFT of g(t), 0.2s Sampling interval')
xlabel('Frequency (Hz)')
The two plots are shown in Figure 8.7.
Figure 8.7 DFT of
g (t )
(d) From Figure 8.7, it can be seen that with the sample interval of 0.05 s,
there was no aliasing and spectrum of G[ k ] in part (b) is almost the same
© 1999 CRC Press LLC
G ( f ) of part (a). With the sampling interval being 0.2 s (less
than the Nyquist rate), there is aliasing and the spectrum of G[ k ] is different from that of G ( f ) .
as that of
Example 8.6
Given a noisy signal
g (t ) = sin(2πf 1 t ) + 0.5n(t )
where
f 1 = 100 Hz
n(t) is a normally distributed white noise. The duration of g ( t ) is 0.5 seconds. Use MATLAB function rand to generate the noise signal.
Use
MATLAB to obtain the power spectral density of g ( t ) .
Solution
A representative program that can be used to plot the noisy signal and obtain
the power spectral density is
MATLAB Script
% power spectral estimation of noisy signal
t = 0.0:0.002:0.5;
f1 =100;
% generate the sine portion of signal
x = sin(2*pi*f1*t);
% generate a normally distributed white noise
n = 0.5*randn(size(t));
% generate the noisy signal
y = x+n;
subplot(211), plot(t(1:50),y(1:50)),
title('Nosiy time domain signal')
% power spectral estimation is done
yfft = fft(y,256);
© 1999 CRC Press LLC
len = length(yfft);
pyy = yfft.*conj(yfft)/len;
f = (500./256)*(0:127);
subplot(212), plot(f,pyy(1:128)),
title('power spectral density'),
xlabel('frequency in Hz')
The plot of the noisy signal and its spectrum is shown in Figure 8.8. The amplitude of the noise and the sinusoidal signal can be changed to observe their
effects on the spectrum.
Figure 8.8 Noisy Signal and Its Spectrum
SELECTED BIBLIOGRAPHY
© 1999 CRC Press LLC
1.
Math Works Inc., MATLAB, High Performance Numeric
Computation Software, 1995.
2.
Etter, D. M., Engineering Problem Solving with MATLAB, 2nd
Edition, Prentice Hall, 1997.
3.
Nilsson, J. W., Electric Circuits, 3rd Edition, Addison-Wesley
Publishing Company, 1990.
4.
Johnson, D. E., Johnson, J.R., and Hilburn, J.L., Electric Circuit
Analysis, 3rd Edition, Prentice Hall, 1997.
EXERCISES
8.1
The triangular waveform, shown in Figure P8.1 can be expressed as
8 A ∞ ( −1) n +1
cos((2n − 1) w0 t )
g (t ) = 2 ∑ 2
π n =1 4 n − 1
where
w0 =
1
Tp
g(t)
A
2Tp
Tp
-A
Figure P8.1 Triangular Waveform
© 1999 CRC Press LLC
If
A = 1, T = 8 ms, and sampling interval is 0.1 ms.
(a)
Write MATLAB program to resynthesize
g (t ) if 20
terms are used.
(b)
What is the root-mean-squared value of the function that is
the difference between g ( t ) and the approximation to
g (t ) when 20 terms are used for the calculation of g (t ) ?
A periodic pulse train
8.2
g (t ) is shown in Figure P8.2.
g(t)
4
1
0
2
3
4
5
6
7
8
t(s)
Figure P8.2 Periodic Pulse Train
If g ( t ) can be expressed by Equation (8.3) ,
(a)
Derive expressions for determining the Fourier Series coefficients an and bn .
8.3
© 1999 CRC Press LLC
(b)
Write a MATLAB program to obtain an and bn for n = 0 ,
1, ......, 10 by using Equations (8.5) and (8.6).
(c)
Resynthesis g(t) using 10 terms of the values an , bn
obtained from part (b).
For the half-wave rectifier waveform, shown in Figure P8.3, with a
period of 0.01 s and a peak voltage of 17 volts.
(a)
Write a MATLAB program to obtain the exponential
Fourier series coefficients cn for n = 0, 1, ......., 20.
(b)
Plot the amplitude spectrum.
(c)
Using the values obtained in (a), use MATLAB to
regenerate the approximation to g ( t ) when 20 terms of the
exponential Fourier series are used.
Figure P8.3 Half-Wave Rectifier Waveform
Figure P8.4(a) is a periodic triangular waveform.
8.4
v(t)
2
-2
0
2
4
6
Figure P8.4(a) Periodic Triangular Waveform
(a)
Derive the Fourier series coefficients an and bn .
(b)
With the signal
v (t ) of the circuit shown in P8.4(b),
i (t ) .
derive the expression for the current
© 1999 CRC Press LLC
t(s)
4H
i(t)
VL(t)
3Ω
V(t)
VR(t)
Figure P8.4(b) Simple RL Circuit
v R ( t ), v L (t ) and also the sum of
v R (t ) and v L ( t ).
(c)
Plot the voltages
(d)
8.5
Compare the voltages of
v R (t ) + v L (t ) to V(t).
If the periodic waveform shown in Figure 8.5 is the input of the
circuit shown in Figure P8.5.
(a)
Derive the mathematical expression for v C ( t ).
(b)
Use MATLAB to plot the signals
g (t ) and v C ( t ).
8Ω
g(t)
4Ω
Figure P8.5 RC Circuit
© 1999 CRC Press LLC
2F
VC(t)
8.6
The unit sample response of a filter is given as
h[n] = (0 − 1 − 1 0 1 1 0)
(a)
Find the discrete Fourier transform of
the values of
(b)
h[n] ; assume that
h[n] not shown are zero.
If the input to the filter is
n
x[n] = sin u[n] , find the
8
output of the filter.
8.7
g (t ) = sin(200πt ) + sin(400πt )
(a)
Generate 512 points of
g (t ). Using the FFT algorithm,
generate and plot the frequency content of g ( t ) .
Assume a sampling rate of 1200 Hz. Find the power
spectrum.
(b)
8.8
Verify that the frequencies in
FFT plot.
g (t ) are observable in the
Find the DFT of
g (t ) = e −5t u(t )
© 1999 CRC Press LLC
g (t ) .
(a)
Find the Fourier transform of
(b)
Find the DFT of g ( t ) using the sampling interval of 0.01 s
and time duration of 5 seconds.
(c)
Compare the results obtained from parts (a) and (b).
CHAPTER NINE
DIODES
In this chapter, the characteristics of diodes are presented. Diode circuit
analysis techniques will be discussed. Problems involving diode circuits are
solved using MATLAB.
9.1
DIODE CHARACTERISTICS
Diode is a two-terminal device. The electronic symbol of a diode is shown in
Figure 9.1(a). Ideally, the diode conducts current in one direction. The current versus voltage characteristics of an ideal diode are shown in Figure 9.1(b).
anode
cathode
i
(a)
i
v
(b)
Figure 9.1 Ideal Diode (a) Electronic Symbol
(b) I-V Characteristics
The I-V characteristic of a semiconductor junction diode is shown in Figure
9.2. The characteristic is divided into three regions: forward-biased, reversedbiased, and the breakdown.
© 1999 CRC Press LLC
i
breakdown
reversedbiased
forwardbiased
0
v
Figure 9.2 I-V Characteristics of a Semiconductor Junction Diode
In the forward-biased and reversed-biased regions, the current, i, and the
voltage, v, of a semiconductor diode are related by the diode equation
i = I S [e ( v / nVT ) − 1]
(9.1)
where
IS
n
VT
is reverse saturation current or leakage current,
is an empirical constant between 1 and 2,
is thermal voltage, given by
VT =
kT
q
and
k
q
T
138
. x10 −23 J / oK,
. x10 −19 Coulombs,
is the electronic charge = 16
is Boltzmann’s constant =
is the absolute temperature in oK
At room temperature (25 oC), the thermal voltage is about 25.7 mV.
© 1999 CRC Press LLC
(9.2)
9.1.1
Forward-biased region
In the forward-biased region, the voltage across the diode is positive. If we
assume that the voltage across the diode is greater than 0.1 V at room
temperature, then Equation (9.1) simplifies to
i = I S e ( v / nVT )
(9.3)
For a particular operating point of the diode (
i D = I S e ( v D / nVT )
i = I D and v = VD ), we have
(9.4)
To obtain the dynamic resistance of the diode at a specified operating point, we
differentiate Equation (9.3) with respect to v, and we have
di I s e ( v / nVT )
=
dv
nVT
di
dv
v =VD
I s e ( v D / nVT )
I
=
= D
nVT
nVT
and the dynamic resistance of the diode, rd , is
rd =
dv
di
v =VD
=
nVT
ID
(9.5)
From Equation (9.3), we have
i
= e ( v / nVT )
IS
thus
ln( i ) =
v
+ ln( I S )
nVT
(9.6)
Equation (9.6) can be used to obtain the diode constants n and I S , given the
data that consists of the corresponding values of voltage and current. From
© 1999 CRC Press LLC
1
nVT
and y-intercept of ln( I S ) . The following example illustrates how to find n
and I S from an experimental data. Since the example requires curve fitting,
Equation (9.6), a curve of
v versus ln(i ) will have a slope given by
the MATLAB function polyfit will be covered before doing the example.
9.1.2
MATLAB function polyfit
The polyfit function is used to compute the best fit of a set of data points to a
polynomial with a specified degree. The general form of the function is
coeff _ xy = polyfit ( x , y, n )
(9.7)
where
x and y are the data points.
n is the n th degree polynomial that will fit the vectors x and y.
coeff _ xy is a polynomial that fits the data in vector y to x in the
least square sense. coeff _ xy returns n+1 coefficients in descending powers of x.
Thus, if the polynomial fit to data in vectors
x and y is given as
coeff _ xy ( x ) = c1 x n + c2 x n−1 + ... + cm
The degree of the polynomial is n and the number of coefficients m = n + 1
and the coefficients ( c1 , c2 , ..., c m ) are returned by the MATLAB polyfit
function.
Example 9.1
A forward-biased diode has the following corresponding voltage and current.
Use MATLAB to determine the reverse saturation current, I S and diode parameter n.
© 1999 CRC Press LLC
Forward Voltage, V
Forward Current, A
0.1
0.133e-12
0.2
1.79e-12
0.3
24.02e-12
0.4
0.321e-9
0.5
4.31e-9
0.6
57.69e-9
0.7
7.726e-7
Solution
diary ex9_1.dat
% Diode parameters
vt = 25.67e-3;
v = [0.1 0.2 0.3 0.4 0.5 0.6 0.7];
i = [0.133e-12 1.79e-12 24.02e-12 321.66e-12 4.31e-9 57.69e-9
772.58e-9];
%
lni = log(i); % Natural log of current
% Coefficients of Best fit linear model is obtained
p_fit = polyfit(v,lni,1);
% linear equation is y = m*x + b
b = p_fit(2);
m = p_fit(1);
ifit = m*v + b;
% Calculate Is and n
Is = exp(b)
n = 1/(m*vt)
% Plot v versus ln(i), and best fit linear model
plot(v,ifit,'w', v, lni,'ow')
axis([0,0.8,-35,-10])
© 1999 CRC Press LLC
xlabel('Voltage (V)')
ylabel('ln(i)')
title('Best fit linear model')
diary
The results obtained from MATLAB are
Is =
9.9525e-015
n =
1.5009
Figure 9.3 shows the best fit linear model used to determine the reverse saturation current, I S , and diode parameter, n.
Figure 9.3 Best Fit Linear Model of Voltage versus Natural
Logarithm of Current
© 1999 CRC Press LLC
9.1.3
Temperature effects
From the diode equation (9.1), the thermal voltage and the reverse saturation
current are temperature dependent. The thermal voltage is directly proportional to temperature. This is expressed in Equation (9.2). The reverse saturation current I S increases approximately 7.2% /oC for both silicon and germanium diodes. The expression for the reverse saturation current as a function of
temperature is
I S (T2 ) = I S (T1 )e [ k S ( T2 − T1 )]
(9.8)
where
k S = 0.072 / o C.
T1 and T2 are two different temperatures.
0.72
is approximately equal to 2, Equation (9.8) can be simplified and
Since e
rewritten as
I S (T2 ) = I S (T1 )2 ( T2 − T1 )/10
(9.9)
Example 9.2
The saturation current of a diode at 25 oC is 10 -12 A. Assuming that the
emission constant of the diode is 1.9, (a) Plot the i-v characteristic of the diode at the following temperatures: T1 = 0 oC, T2 = 100 oC.
Solution
MATLAB Script
% Temperature effects on diode characteristics
%
k = 1.38e-23; q = 1.6e-19;
t1 = 273 + 0;
t2 = 273 + 100;
ls1 = 1.0e-12;
ks = 0.072;
ls2 = ls1*exp(ks*(t2-t1));
v = 0.45:0.01:0.7;
© 1999 CRC Press LLC
l1 = ls1*exp(q*v/(k*t1));
l2 = ls2*exp(q*v/(k*t2));
plot(v,l1,'wo',v,l2,'w+')
axis([0.45,0.75,0,10])
title('Diode I-V Curve at two Temperatures')
xlabel('Voltage (V)')
ylabel('Current (A)')
text(0.5,8,'o is for 100 degrees C')
text(0.5,7, '+ is for 0 degree C')
Figure 9.4 shows the temperature effects of the diode forward characteristics.
Figure 9.4 Temperature Effects on the Diode Forward
Characteristics
© 1999 CRC Press LLC
9.2
ANALYSIS OF DIODE CIRCUITS
Figure 9.5 shows a diode circuit consisting of a dc source
and a diode. We want to determine the diode current
age
VDC , resistance R ,
I D and the diode volt-
VD .
R
ID
+
VDC
-
+
VD
-
Figure 9.5 Basic Diode Circuit
Using Kirchoff Voltage Law, we can write the loadline equation
VDC = RI D + VD
(9.10)
The diode current and voltage will be related by the diode equation
i D = I S e ( v D / nVT )
Equations (9.10) and (9.11) can be used to solve for the current
age
(9.11)
I D and volt-
VD .
There are several approaches for solving I D and VD . In one approach,
Equations (9.10) and (9.11) are plotted and the intersection of the linear curve
of Equation (9.10) and the nonlinear curve of Equation (9.11) will be the operating point of the diode. This is illustrated by the following example.
© 1999 CRC Press LLC
Example 9.3
For the circuit shown in Figure 9.5, if R = 10 kΩ , V DC = 10V, and the
reverse saturation current of the diode is 10 -12 A and n = 2.0. (Assume a
temperature of 25 oC.)
(a)
Use MATLAB to plot the diode forward characteristic curve and the
loadline.
(b)
From the plot estimate the operating point of the diode.
Solution
MATLAB Script
% Determination of operating point using
% graphical technique
%
% diode equation
k = 1.38e-23;q = 1.6e-19;
t1 = 273 + 25; vt = k*t1/q;
v1 = 0.25:0.05:1.1;
i1 = 1.0e-12*exp(v1/(2.0*vt));
% load line 10=(1.0e4)i2 + v2
vdc = 10;
r = 1.0e4;
v2 = 0:2:10;
i2 = (vdc - v2)/r;
% plot
plot(v1,i1,'w', v2,i2,'w')
axis([0,2, 0, 0.0015])
title('Graphical method - operating point')
xlabel('Voltage (V)')
ylabel('Current (A)')
text(0.4,1.05e-3,'Loadline')
text(1.08,0.3e-3,'Diode curve')
Figure 9.6 shows the intersection of the diode forward characteristics and the
loadline.
© 1999 CRC Press LLC
Figure 9.6 Loadline and Diode Forward Characteristics
From Figure 9.6, the operating point of the diode is the intersection of the
loadline and the diode forward characteristic curve. The operating point is approximately
I D = 0.9 mA
V D = 0.7 V
The second approach for obtaining the diode current
VD
(I
D2
of Figure 9.5 is to use
iteration.
I D and diode voltage
Assume that
(I
D1
, VD1 ) and
, VD 2 ) are two corresponding points on the diode forward characteris-
tics. Then, from Equation (9.3), we have
© 1999 CRC Press LLC
i D1 = I S e ( v D1 / nVT )
(9.12)
i D 2 = I S e ( v D 2 / nVT )
(9.13)
Dividing Equation (9.13) by (9.12), we have
I D2
= e (VD 2 −VD1 / nVT )
I D1
(9.14)
Simplifying Equation (9.14), we have
I 2
v D 2 = v D1 + nVT ln D
I D1
(9.15)
Using iteration, Equation (9.15) and the loadline Equation (9.10) can be used
to obtain the operating point of the diode.
To show how the iterative technique is used, we assume that
I D1 = 1mA and
VD1 = 0.7 V. Using Equation (9.10), I D2 is calculated by
I D2 =
VDC − VD1
R
Using Equation (9.15),
V D2 is calculated by
I 2
VD 2 = VD1 + nVT ln D
I D1
Using Equation (9.10),
I D3 =
(9.16)
(9.17)
I D3 is calculated by
V DC − V D 2
R
(9.18)
Using Equation (9.15) , VD3 is calculated by
I 3
VD 3 = V D1 + nVT ln D
I D1
Similarly,
© 1999 CRC Press LLC
I D4 and V D4 are calculated by
(9.19)
I D4 =
VDC − VD 3
R
(9.20)
V D 4 = VD1 + nVT ln(
The iteration is stopped when
I D4
)
I D1
(9.21)
VDn is approximately equal to V Dn−1 or I Dn
is approximately equal to I Dn−1 to the desired decimal points. The iteration
technique is particularly facilitated by using computers. Example 9.4 illustrates the use of MATLAB for doing the iteration technique.
Example 9.4
Redo Example 9.3 using the iterative technique. The iteration can be stopped
when the current and previous value of the diode voltage are different by
volts.
Solution
MATLAB Script
% Determination of diode operating point using
% iterative method
k = 1.38e-23;q = 1.6e-19;
t1 = 273 + 25; vt = k*t1/q;
vdc = 10;
r = 1.0e4;
n = 2;
id(1) = 1.0e-3; vd(1) = 0.7;
reltol = 1.0e-7;
i = 1;
vdiff = 1;
while vdiff > reltol
id(i+1) = (vdc - vd(i))/r;
vd(i+1) = vd(i) + n*vt*log(id(i+1)/id(i));
vdiff = abs(vd(i+1) - vd(i));
i = i+1;
end
k = 0:i-1;
% operating point of diode is (vdiode, idiode)
idiode = id(i)
© 1999 CRC Press LLC
10 −7
vdiode = vd(i)
% Plot the voltages during iteration process
plot(k,vd,'wo')
axis([-1,5,0.6958,0.701])
title('Diode Voltage during Iteration')
xlabel('Iteration Number')
ylabel('Voltage, V')
From the MATLAB program, we have
idiode =
9.3037e-004
vdiode =
0.6963
Thus I D = 0.9304 mA and V D =
voltage during the iteration process.
0.6963 V. Figure 9.7 shows the diode
Figure 9.7 Diode Voltage during Iteration Process
© 1999 CRC Press LLC
9.3
HALF-WAVE RECTIFIER
A half-wave rectifier circuit is shown in Figure 9.8. It consists of an alternating current (ac) source, a diode and a resistor.
+
+
R
Vo
Vs
-
-
Figure 9.8 Half-wave Rectifier Circuit
Assuming that the diode is ideal, the diode conducts when source voltage is
positive, making
v0 = v S
when
vS ≥ 0
(9.22)
When the source voltage is negative, the diode is cut-off, and the output voltage is
v0 = 0
when
vS < 0
(9.23)
Figure 9.9 shows the input and output waveforms when the input signal is a
sinusoidal signal.
The battery charging circuit, explored in the following example, consists of a
source connected to a battery through a resistor and a diode.
© 1999 CRC Press LLC
Figure 9.9 (a) Input and (b) Output Waveforms of a Half-wave
Rectifier Circuit
Example 9.5
A battery charging circuit is shown in Figure 9.10. The battery voltage is
. V. The source voltage is v S (t ) = 18 sin(120πt ) V and the
V B = 118
resistance is R = 100 Ω. Use MATLAB (a) to sketch the input voltage, (b)
to plot the current flowing through the diode, (c ) to calculate the conduction
angle of the diode, and (d) calculate the peak current. (Assume that the diode
is ideal.)
R
iD
+
Vs
-
Figure 9.10 A Battery Charging Circuit
© 1999 CRC Press LLC
VB
Solution:
When the input voltage
ode current,
v S is greater than VB , the diode conducts and the di-
i d , is given as
id =
VS − V B
R
The diode starts conducting at an angle θ, given by
(9.24)
v S ≥ VB , i.e.,
18 sin θ1 = 18 sin(120πt1 ) = VB = 118
.
The diode stops conducting current when vs ≤ VB
18 sin θ2 = 18 sin(120πt 2 ) = V B
due to the symmetry
θ2 = π − θ1
MATLAB Program:
diary ex9_5.dat
% Baltery charging circuit
period = 1/60;
period2 = period*2;
inc =period/100;
npts = period2/inc;
vb = 11.8;
t = [];
for i = 1:npts
t(i) = (i-1)*inc;
vs(i) = 18*sin(120*pi*t(i));
if vs(i) > vb
idiode(i) = (vs(i) -vb)/r;
else
idiode(i) = 0;
end
end
© 1999 CRC Press LLC
subplot(211), plot(t,vs)
%title('Input Voltage')
xlabel('Time (s)')
ylabel('Voltage (V)')
text(0.027,10, 'Input Voltage')
subplot(212), plot(t,idiode)
%title('Diode Current')
xlabel('Time (s)')
ylabel('Current(A)')
text(0.027, 0.7e-3, 'Diode Current')
% conduction angle
theta1 = asin(vb/18); theta2 = pi - theta1;
acond = (theta2 -theta1)/(2*pi)
% peak current
pcurrent = (18*sin(pi/2) - vb)/r
% pcurrent = max(idiode)
diary
The conduction angle, acond, and the peak current, pcurrent, are
acond =
0.2724
pcurrent =
0.0620
Figure 9.11 shows the input voltage and diode current.
The output of the half-wave rectifier circuit of Figure 9.8 can be smoothed by
connecting a capacitor across the load resistor. The smoothing circuit is shown
in Figure 9.12.
When the amplitude of the source voltage VS is greater than the output voltage, the diode conducts and the capacitor is charged. When the source voltage
becomes less than the output voltage, the diode is cut-off and the capacitor
discharges with the time constant CR. The output voltage and the diode current waveforms are shown in Figure 9.13.
© 1999 CRC Press LLC
Figure 9.11 Input Voltage and Diode Current
id
+
Vs
-
Figure 9.12 Capacitor Smoothing Circuit
© 1999 CRC Press LLC
+
R
C
Vo
-
Vo
Vm
t1
t2
t3
t4
t
t3
t4
t
T
iD
t1
t2
Figure 9.13
(a) Output Voltage and (b) Diode Current for Halfwave Rectifier with Smoothing Capacitor Filter
In Figure 9.12(a), the output voltage reaches the maximum voltage
Vm , at
t = t 2 to t = t 3 , the diode conduction ceases, and capacitor discharges
through R. The output voltage between times t 2 and t 3 is given as
time
v 0 (t ) = Vm e
t −t2
−
RC
t2 < t < t3
(9.25)
The peak to peak ripple voltage is defined as
Vr = v 0 (t 2 ) − v 0 (t 3 ) = Vm − Vm e
t −t
− 3 2
= Vm 1 − e RC
For large values C such that CR >>
(9.26)
(t
3
ponential series approximation
e−x ≅ 1 − x
© 1999 CRC Press LLC
t −t
− 3 2
RC
for x << 1
− t 2 ) , we can use the well-known ex-
Thus, Equation (9.26) approximates to
Vr =
Vm (t 3 − t 2 )
RC
(9.27)
The discharging time for the capacitor,
(t
3
− t 2 ) , is approximately equal to
the period of the input ac signal, provided the time constant is large. That is,
t3 − t2 ≅ T =
1
f0
(9.28)
where
f 0 is the frequency of the input ac source voltage.
Using Equation (9.28), Equation (9.27) becomes
Vr ( peak − to − peak ) =
Vm
f 0 CR
(9.29)
For rectifier circuits, because RC >> T , the output voltage decays for a small
fraction of its fully charged voltage, and the output voltage may be regarded as
linear. Therefore, the output waveform of Figure 9.12 is approximately triangular. The rms value of the triangular wave is given by
Vrms =
V peak − to − peak
2 3
=
Vm
2 3 f o CR
(9.30)
The approximately dc voltage of the output waveform is
Vdc = Vm −
9.3.1
Vr
Vm
= Vm −
2
2 f o CR
(9.31)
MATLAB function fzero
The MATLAB fzero is used to obtain the zero of a function of one variable.
The general form of the fzero function is
© 1999 CRC Press LLC
fzero(' function', x1)
fzero(' function', x1, tol )
where
fzero(' funct ', x1) finds the zero of the function funct ( x ) that
is near the point x1 .
fzero(' funct ', x1, tol ) returns zero of the function funct ( x )
accurate to within a relative error of tol.
The MATLAB function fzero is used in the following example.
Example 9.6
For a capacitor smoothing circuit of Figure 9.12, if R = 10KΩ, C = 100µF,
and
v S (t ) = 120 2 sin(120πt ) ,
(a) use MATLAB to calculate the times t 2 , t 3 , of Figure 9.12;
(b) compare the capacitor discharge time with period of the input signal.
Solution
The maximum value of
v S (t ) is 120 2 , and it occurs at 120πt 2 =
thus
t2 =
1
= 0.00417 s
240
The capacitor discharge waveform is given by
(t − t 2 )
v C (t ) = 120 2 exp −
RC
At
© 1999 CRC Press LLC
t = t3
v C (t ) = v S (t ) ,
t2 < t < t3
π
,
2
Defining
v ( t ) as
(
)
(t − t 2 )
v (t ) = 120 2 sin 120π (t − t p ) − 120 2 exp −
RC
Then,
(t − t )
v (t 3 ) = 0 = 120 2 sin 120π (t 3 − t p ) − 120 2 exp − 3 2
RC
Thus,
(
)
(
)
(t 3 − t 2 )
v (t 3 ) = 0 = sin 120π (t 3 − t p ) − exp −
RC
MATLAB is used to solve Equation (9.32)
MATLAB Script
diary ex9_6.dat
% Capacitance discharge time for smoothing capacitor
% filter circuit
vm = 120*sqrt(2);
f0 = 60; r =10e3; c = 100e-6;
t2 = 1/(4*f0);
tp = 1/f0;
% use MATLAB function fzero to find the zero of a
% function of one variable
rc = r*c;
t3 = fzero('sinexpf1',4.5*t2);
tdis_cap = t3- t2;
fprintf('The value of t2 is %9.5f s\n', t2)
fprintf('The value of t3 is %9.5f s\n', t3)
fprintf('The capacitor discharge time is %9.5f s\n', tdis_cap)
fprintf('The period of input signal is %9.5f s\n', tp)
diary
%
function y = sinexpf1(t)
t2 = 1/240; tp = 1/60;
rc = 10e3*100e-6;
y = sin(120*pi*(t-tp)) - exp(-(t-t2)/rc);
end
The results are
© 1999 CRC Press LLC
(9.32)
The value of t2 is 0.00417 s
The value of t3 is 0.02036 s
The capacitor discharge time is 0.01619 s
The period of input signal is 0.01667 s
9.4
FULL-WAVE RECTIFICATION
A full-wave rectifier that uses a center-tapped transformer is shown in Figure
9.14.
D1
A
+
+
Vs(t)
R
+
Vo(t)
-
Vs(t)
D2
Figure 9.14 Full-wave Rectifier Circuit with Center-tapped
Transformer
v S (t ) is positive, the diode D1 conducts but diode D2 is off, and the
output voltage v 0 ( t ) is given as
When
v 0 ( t ) = v S (t ) − V D
(9.33)
where
VD is a voltage drop across a diode.
v S (t ) is negative, diode D1 is cut-off but diode D2 conducts. The
current flowing through the load R enters it through node A. The output volt-
When
age is
v (t ) = v S (t ) − V D
© 1999 CRC Press LLC
(9.34)
A full-wave rectifier that does not require a center-tapped transformer is the
bridge rectifier of Figure 9.15.
D4
Vs(t)
D1
A
D3
D2
R
Vo(t)
Figure 9.15 Bridge Rectifier
v S (t ) is negative, the diodes D2 and D4 conduct, but diodes D1 and
D3 do not conduct. The current entering the load resistance R enters it
When
through node A. The output voltage is
v (t ) = v S (t ) − 2VD
(9.35)
Figure 9.16 shows the input and output waveforms of a full-wave rectifier circuit assuming ideal diodes.
The output voltage of a full-wave rectifier circuit can be smoothed by connecting a capacitor across the load. The resulting circuit is shown in Figure 9.17.
The output voltage and the current waveforms for the full-wave rectifier with
RC filter are shown in Figure 9.18.
© 1999 CRC Press LLC
Figure 9.16
(a) Input and (b) Output Voltage Waveforms for Fullwave Rectifier Circuit
D4
Vs(t)
D1
A
D3
D2
R
C
Vo(t)
Figure 9.17 Full-wave Rectifier with Capacitor Smoothing Filter
© 1999 CRC Press LLC
Vo(t)
Vm
t
(a)
i
t1
t2
t3
t4
t5
t6
t7
t8
t
(b)
Figure 9.18
(a) Voltage and (b) Current Waveform of a Full-wave
Rectifier with RC Filter
From Figures 9.13 and 9.18, it can be seen that the frequency of the ripple
voltage is twice that of the input voltage. The capacitor in Figure 9.17 has
only half the time to discharge. Therefore, for a given time constant, CR, the
ripple voltage will be reduced, and it is given by
Vr ( peak − to − peak ) =
Vm
2 f o CR
(9.36)
where
Vm
f0
is peak value of the input sinusoidal waveform
frequency of the input sinusoidal waveform
The rms value of the ripple voltage is
Vrms =
Vm
4 3 f o CR
and the output dc voltage is approximately
© 1999 CRC Press LLC
(9.37)
Vdc = Vm −
Vr
Vm
= Vm −
2
4 f o CR
(9.38)
Example 9.7
For the full-wave rectifier with RC filter shown in Figure 9.17, if
v S (t ) = 20 sin(120πt ) and R = 10KΩ, C = 100µF, use MATLAB to find
the
(a) peak-to-peak value of ripple voltage,
(b) dc output voltage,
(c) discharge time of the capacitor,
(d) period of the ripple voltage.
Solution
Peak-to-peak ripple voltage and dc output voltage can be calculated using
Equations (9.36) and (9.37), respectively. The discharge time of the capacitor
is the time
(t
3
− t 1 ) of Figure 9.19.
V2(t)
V1(t)
Vm
t1
t2
t3
t4
Figure 9.19 Diagram for Calculating Capacitor Discharge Time
(t − t1 )
v1 (t ) = Vm exp −
RC
(9.39)
v 2 (t ) = Vm sin[2π (t − t 2 )]
(9.40)
v1 (t ) and v 2 (t ) intersect at time t 3 .
The period of input waveform,
© 1999 CRC Press LLC
v S (t ) is T =
1
s
240
Thus,
t1 =
1
T
s,
=
4 240
and
t2 =
1
T
s
=
2 120
MATLAB Script
diary ex9_7.dat
% Full-wave rectifier
%
period = 1/60;
t1 = period/4;
vripple = 20/(2*60*10e3*100e-6);
vdc = 20 - vripple/2;
t3 = fzero('sinexpf2',0.7*period);
tdis_cap = t3 - t1;
fprintf('Ripple value (peak-peak) is %9.5f V\n', vripple)
fprintf('DC output voltage is %9.5f V\n', vdc)
fprintf('Capacitor discharge time is %9.5f s\n', tdis_cap)
fprintf('Period of ripple voltage is %9.5f s\n', 0.5*period)
diary
%
%
function y = sinexpf2(t)
t1 = 1/240; t2 = 2*t1; rc = 10e3*100e-6;
y = 20(sin(120*pi*(t - t2))) - exp(-(t-t1)/rc);
end
The results are
Ripple value (peak-peak) is 0.16667 V
DC output voltage is 19.91667 V
Capacitor discharge time is 0.00800 s
Period of ripple voltage is 0.00833 s
© 1999 CRC Press LLC
(9.41)
9.5
ZENER DIODE VOLTAGE REGULATOR CIRCUITS
The zener diode is a pn junction diode with controlled reverse-biased breakdown voltage. Figure 9.20 shows the electronic symbol and the current-voltage
characteristics of the zener diode.
i
v
(a)
i
Vz
Izk
v
slope = 1/rz
Izm
(b)
Figure 9.20 Zener Diode (a) Electronic Symbol (b) I-V
Characteristics
I ZK is the minimum current needed for the zener to breakdown. I ZM is the
maximum current that can flow through the zener without being destroyed. It
is obtained by
I ZM =
where
PZ
VZ
(9.42)
PZ is the zener power dissipation.
The incremental resistance of the zener diode at the operating point is specified
by
© 1999 CRC Press LLC
∆VZ
∆I Z
rZ =
(9.43)
One of the applications of a zener diode is its use in the design of voltage reference circuits. A zener diode shunt voltage regulator circuit is shown in Figure 9.21
Rs
Is
Vs
Iz
Vz
Il
+
Rl
Vo
-
Figure 9.21 Zener Diode Shunt Voltage Regulator Circuit
The circuit is used to provide an output voltage, V0 , which is nearly constant.
When the source voltage is greater than the zener breakdown voltage, the zener
will break down ` and the output voltage will be equal to the zener breakdown
voltage. Thus,
V0 = VZ
(9.44)
From Kirchoff current law, we have
IS = IZ + I L
(9.45)
and from Ohm’s Law, we have
IS =
VS − VZ
RS
(9.46)
IL =
VO
RL
(9.47)
and
© 1999 CRC Press LLC
RL is held constant and VS (which was originally greater than VZ ) is increased, the source current I S will increase; and
since I L is constant, the current flowing through the zener will increase. Conversely, if R is constant and VS decreases, the current flowing through the
Assuming the load resistance
zener will decrease since the breakdown voltage is nearly constant; the output
voltage will remain almost constant with changes in the source voltage VS .
Now assuming the source voltage is held constant and the load resistance is
decreased, then the current I L will increase and I Z will decrease. Con-
VS is held constant and the load resistance increases, the current
through the load resistance I L will decrease and the zener current I Z will
versely, if
increase.
In the design of zener voltage regulator circuits, it is important that the zener
diode remains in the breakdown region irrespective of the changes in the load
or the source voltage. There are two extreme input/output conditions that will
be considered:
I Z is minimum when the load current I L is
maximum and the source voltage VS is minimum.
(1)
The diode current
(2)
The diode current
I Z is maximum when the load current I L is
minimum and the source voltage VS is maximum.
From condition (1) and Equation (9.46), we have
RS =
VS ,min − VZ
I L ,max + I Z ,min
(9.48)
Similarly, from condition (2), we get
RS =
VS ,max − VZ
I L ,min + I Z ,max
(9.49)
Equating Equations (9.48) and (9.49) , we get
(VS ,min − VZ )( I L ,min + I Z ,max ) = (VS ,max − VZ )( I L ,max + I Z ,min )
© 1999 CRC Press LLC
(9.50)
We use the rule of thumb that the maximum zener current is about ten times
the minimum value, that is
. I Z ,max
I Z ,min = 01
(9.51)
Substituting Equation (9.49) into Equation (9.51), and solving for
I Z,max , we
obtain
I Z ,max =
I L ,min (VZ − VS ,min ) + I L ,max (VS ,max − VZ )
VS ,min − 0.9VZ − 0.1VS ,max
(9.52)
I Z,max , we can use Equation (9.49) to calculate RS . The following
Knowing
example uses MATLAB to solve a zener voltage regulator problem.
Example 9.8
A zener diode voltage regulator circuit of Figure 9.21 has the following data:
30 ≤
VS ≤ 35V; R L = 10K, RS = 2K
VZ = −20 + 0.05I
for -100 mA ≤ I < 0
(9.53)
Use MATLAB to
(a) plot the zener breakdown characteristics, (b) plot the loadline for
30V and
VS =
VS = 35 V, (c) determine the output voltage when VS = 30V and
VS = 35V.
Solution
Using Thevenin Theorem, Figure 9.21 can be simplified into the form shown
in Figure 9.22.
© 1999 CRC Press LLC
RT
I
VT
Vz
-
Figure 9.22 Equivalent Circuit of Voltage Regulator Circuit
VT =
VS R L
R L + RS
(9.54)
and
RT = R L RS
Since
(9.55)
R L = 10K, RS = 2K, RT = (10)(2K) / 12 K = 1.67 KΩ
when VS = 30V,
VT = (30)(10) / 12 = 25 V
when VS = 35V,
VT = (35)(10) / 12 = 29.17 V
The loadline equation is
VT = RT I + VZ
Equations (9.53) and (9.56) are two linear equations solving for
(9.56)
I, so we get
VZ = VT − RT I = −20 + 0.05I
⇒
© 1999 CRC Press LLC
I=
(VT + 20)
RT + 0.05
(9.57)
From Equations (9.56) and (9.57), the output voltage (which is also zener voltage) is
VZ = VT − RT I = VT −
RT (VT + 20)
RT + 0.05
MATLAB program
diary ex9_8.dat
% Zener diode voltage regulator
vs1 = -30; vs2 = -35; rl =10e3; rs = 2e3;
i = -50e-3: 5e-3 :0;
vz = -20 + 0.05*i;
m = length(i);
i(m+1) = 0; vz(m+1) = -10;
i(m+2) = 0; vz(m+2) = 0;
% loadlines
vt1 = vs1*rl/(rl+rs);
vt2 = vs2*rl/(rl+rs);
rt = rl*rs/(rl+rs);
l1 = vt1/20;
l2 = vt2/20;
v1 = vt1:abs(l1):0;
i1 = (vt1 - v1)/rt;
v2 = vt2:abs(l2):0;
i2 = (vt2 - v2)/rt;
% plots of Zener characteristics, loadlines
plot(vz,i,'w',v1,i1,'w',v2,i2,'w')
axis([-30,0,-0.03,0.005])
title('Zener Voltage Regulator Circuit')
xlabel('Voltage (V)')
ylabel('Current (A)')
text(-19.5,-0.025,'Zener Diode Curve')
text(-18.6,-0.016, 'Loadline (35 V Source)')
text(-14.7,-0.005,'Loadline (30 V Source)')
% output voltage when vs = -30v
ip1 = (vt1 + 20)/(rt + 0.05)
vp1 = vt1 - rt*(vt1+20)/(rt + 0.05)
% output voltage when vs = -35v
ip2 = (vt2 + 20)/(rt + 0.05)
vp2 = vt2 - rt*(vt2+20)/(rt + 0.05)
diary
The results obtained are
© 1999 CRC Press LLC
(9.58)
ip1 =
-0.0030
vp1 =
-20.0001
ip2 =
-0.0055
vp2 =
-20.0003
When the source voltage is 30 V, the output voltage is 20.0001 V.
In addition, when the source voltage is 35 V, the output voltage is 20.0003 V.
The zener breakdown characteristics and the loadlines are shown in Figure
9.23.
Figure 9.23 Zener Characteristics and Loadlines
© 1999 CRC Press LLC
SELECTED BIBLIOGRAPHY
1.
Lexton, R. Problems and Solutions in Electronics,
Hall, 1994
Chapman &
2.
Shah, M. M., Design of Electronics Circuits and Computer Aided
Design, John Wiley & Sons, 1993.
3.
Angelo, Jr., E.J., Electronic Circuits, McGraw Hill, 1964.
4.
Sedra, A.S. and Smith, K.C., Microelectronic Circuits, 4th Edition,
Oxford University Press, 1997.
5.
Beards, P.H., Analog and Digital Electronics - A First Course, 2nd
Edition, Prentice Hall, 1990.
6.
Savant, Jr., C.J., Roden, M.S.,and Carpenter, G.L., Electronic Circuit
Design: An Engineering Approach, Benjamin/Cummings Publishing
Co., 1987.
7.
Ferris, C.D., Elements of Electronic Design, West Publishing Co.,
1995.
8.
Ghausi, M.S., Electronic Devices and Circuits: Discrete and
Integrated, Holt, Rinehart and Winston, 1985.
9.
Warner Jr., R.M. and Grung, B.L. Semiconductor Device
Electronics, Holt, Rinehart and Winston, 1991.
EXERCISES
9.1
Use the iteration technique to find the voltage
o
Figure P9.1. Assume that T = 25 C,
iteration when
© 1999 CRC Press LLC
Vn − Vn −1 < 10
−9
VD and the I D
-16
of
n = 1.5, I S = 10 A. Stop current the
V.
4 kilohms
10 V
5.6 kilohms
6 kilohms
ID
VD
Figure P9.1 A Diode Circuit
9.2
A zener diode has the following I-V characteristics
Reverse Voltage (V)
Reverse Current (A)
-2
-1.0e-10
-4
-1.0e-10
-6
-1.0e-8
-8
-1.0e-5
-8.5
-2.0e-5
-8.7
-15.0e-3
-8.9
-43.5 e-3
(a) Plot the reverse characteristics of the diode. (b) What is the
breakdown voltage of the diode? (c ) Determine the dynamic resistance of the diode in its breakdown region.
9.3
A forward-biased diode has the following corresponding voltage and
current.
(a) Plot the static I-V characteristics.
(b) Determine the diode parameters
© 1999 CRC Press LLC
I S and n.
(c) Calculate the dynamic resistance of the diode at VS = 0.5 V.
Forward Voltage, V
Forward Current, A
0.2
7.54e-7
0.3
6.55e-6
0.4
5.69e-5
0.5
4.94e-4
0.6
4.29e-3
0.7
3.73e-2
.
9.4
For Figure P9.4,
10 k Ω
5 kΩ
Id
10 k Ω
20 V
10 k Ω
15 k Ω
Figure P9.4 Diode Circuit
(a) Use iteration to find the current through the diode. The iteration
can be stopped when
I dn − I dn −1 < 10 −12 A.
(b) How many iterations were performed before the required result
was obtained? Assume a temperature of 25 oC, emission coefficient, n , of 1.5, and the reverse saturation current, I S , is 10-16
A.
© 1999 CRC Press LLC
9.5 For a full-wave rectifier circuit with smoothing capacitor shown in
ure 9.17, if v S ( t ) = 100 sin(120πt ) V, R = 50KΩ, C = 250µF,
MATLAB
Figusing
(a) Plot the input and output voltages when the capacitor is disconnected from the load resistance R.
(b) When the capacitor is connected across load resistance R, determine the conduction time of the diode.
(c) What is the diode conduction time?
9.6 For the voltage regulator circuit shown in Figure 9.21, assume that 50 <
VS < 60 V, R L = 50K, RS = 5K, VS = -40 + 0.01 I. Use MATLAB
to
(a) Plot the zener diode breakdown characteristics.
(b) Plot the loadline for VS = 50 V and
VS = 60V.
(c) Determine the output voltage and the current flowing through the
source resistance RS when VS = 50V and VS = 60V.
VS = 35V, RS
9.7 For the zener voltage regulator shown in Figure 9.21, If
= 1KΩ,
VZ = −25 + 0.02 I and 5K < R L < 50K, use MATLAB to
(a) Plot the zener breakdown characteristics
(b) Plot the loadline when
R L = 5K and R L = 50K.
(c) Determine the output voltage when R L = 5KΩ and
(d) What is the power dissipation of the diode when
© 1999 CRC Press LLC
R L = 50KΩ.
R L = 50KΩ?
CHAPTER TEN
SEMICONDUCTOR PHYSICS
In this chapter, a brief description of the basic concepts governing the flow of
current in a pn junction are discussed. Both intrinsic and extrinsic semiconductors are discussed. The characteristics of depletion and diffusion capacitance are explored through the use of example problems solved with
MATLAB. The effect of doping concentration on the breakdown voltage of
pn junctions is examined.
10.1
10.1.1
INTRINSIC SEMICONDUCTORS
Energy bands
According to the planetary model of an isolated atom, the nucleus that contains protons and neutrons constitutes most of the mass of the atom. Electrons
surround the nucleus in specific orbits. The electrons are negatively charged
and the nucleus is positively charged. If an electron absorbs energy (in the
form of a photon), it moves to orbits further from the nucleus. An electron
transition from a higher energy orbit to a lower energy orbit emits a photon for
a direct band gap semiconductor.
1.21 eV gap
valence band
conduction band
0.66 eV gap
valence band
energy of electrons
conduction band
energy of electrons
energy of electrons
The energy levels of the outer electrons form energy bands. In insulators, the
lower energy band (valence band) is completely filled and the next energy
band (conduction band) is completely empty. The valence and conduction
bands are separated by a forbidden energy gap.
conduction band
5.5 eV gap
valence band
Figure 10.1 Energy Level Diagram of (a) Silicon, (b) Germanium,
and (c ) Insulator (Carbon)
© 1999 CRC Press LLC
In conductors, the valence band partially overlaps the conduction band with no
forbidden energy gap between the valence and conduction bands. In semiconductors the forbidden gap is less than 1.5eV. Some semiconductor materials
are silicon (Si), germanium (Ge), and gallium arsenide (GaAs). Figure 10.1
shows the energy level diagram of silicon, germanium and insulator (carbon).
10.1.2
Mobile carriers
Silicon is the most commonly used semiconductor material in the integrated
circuit industry. Silicon has four valence electrons and its atoms are bound together by covalent bonds. At absolute zero temperature the valence band is
completely filled with electrons and no current flow can take place. As the
temperature of a silicon crystal is raised, there is increased probability of
breaking covalent bonds and freeing electrons. The vacancies left by the freed
electrons are holes. The process of creating free electron-hole pairs is called
ionization. The free electrons move in the conduction band. The average
number of carriers (mobile electrons or holes) that exist in an intrinsic semiconductor material may be found from the mass-action law:
ni = AT 1.5 e
[ − E g /( kT )]
(10.1)
where
T is the absolute temperature in oK
k is Boltzmann’s constant ( k = 1.38 x 10-23 J/K or 8.62x10-5
eV/K )
E g is the width of the forbidden gap in eV. E g is 1.21 and
1.1eV for Si at 0oK and 300oK, respectively. It is given as
E g = Ec − Ev
(10.2)
A is a constant dependent on a given material and it is given as
*
m*p 3/ 4
2
3/ 2 mn
)
A = 3 (2πm0 k ) (
m0 mo
h
where
© 1999 CRC Press LLC
(10.3)
h is Planck’s constant (h = 6.62 x 10-34 J s or 4.14 x 10-15 eV s).
mo is the rest mass of an electron
mn* is the effective mass of an electron in a material
mp* is effective mass of a hole in a material
The mobile carrier concentrations are dependent on the width of the energy
gap, E g , measured with respect to the thermal energy kT. For small values
of T ( kT <<
E g ), ni is small implying, there are less mobile carriers.
For silicon, the equilibrium intrinsic concentration at room temperature is
ni = 1.52 x 1010 electrons/cm3
(10.4)
Of the two carriers that we find in semiconductors, the electrons have a higher
mobility than holes. For example, intrinsic silicon at 300oK has electron
mobility of 1350 cm2 / volt-sec and hole mobility of 480 cm2 / volt-sec. The
conductivity of an intrinsic semiconductor is given by
σ i = q (ni µn + pi µ p )
(10.5)
where
q
ni
pi
µn
µp
is the electronic charge (1.6 x 10-19 C)
is the electron concentration
is the hole concentration. pi = ni for the intrinsic
semiconductor
electron mobility in the semiconductor material
hole mobility in the semiconductor material.
Since electron mobility is about three times that of hole mobility in silicon, the
electron current is considerably more than the hole current. The following example illustrates the dependence of electron concentration on temperature.
© 1999 CRC Press LLC
Example 10.1
Given that at T = 300 oK, the electron concentration in silicon is 1.52 x 1010
electrons /cm3 and E g = 1.1 eV at 300 oK.
(a) Find the constant A of Equation (10.1).
(b) Use MATLAB to plot the electron concentration versus temperature.
Solution
From Equation (10.1), we have
152
. x1010 = A(300) 1.5 e[ −1.1/ 300*8.62*10
We use MATLAB to solve for
is given as [1].
−5
)]
A. The width of energy gap with temperature
T2
. − 4.37 x10 − 4
E g (T ) = 117
T + 636
(10.6)
Using Equations (10.1) and (10.6), we can calculate the electron concentration
at various temperatures.
MATLAB Script
%
% Calculation of the constant A
diary ex10_1.dat
k = 8.62e-5;
na = 1.52e10; ta = 300;
ega = 1.1;
ka = -ega/(k*ta);
t32a = ta.^1.5;
A = na/(t32a*exp(ka));
fprintf('constant A is %10.5e \n', A)
% Electron Concentration vs. temperature
for i = 1:10
t(i) = 273 + 10*(i-1);
© 1999 CRC Press LLC
eg(i) = 1.17 - 4.37e-4*(t(i)*t(i))/(t(i) + 636);
t32(i) = t(i).^1.5;
ni(i) = A*t32(i)*exp(-eg(i)/(k*t(i)));
end
semilogy(t,ni)
title('Electron Concentration vs. Temperature')
xlabel('Temperature, K')
ylabel('Electron Concentration, cm-3')
Result for part (a)
constant A is 8.70225e+024
Figure 10.2 shows the plot of the electron concentration versus temperature.
Figure 10.2 Electron Concentration versus Temperature
© 1999 CRC Press LLC
10.2
10.2.1
EXTRINSIC SEMICONDUCTOR
Electron and hole concentrations
Extrinsic semiconductors are formed by adding specific amounts of impurity
atoms to the silicon crystal. An n-type semiconductor is formed by doping the
silicon crystal with elements of group V of the periodic table (antimony, arsenic, and phosphorus). The impurity atom is called a donor. The majority carriers are electrons and the minority carriers are holes. A p-type semiconductor
is formed by doping the silicon crystal with elements of group III of the periodic table (aluminum, boron, gallium, and indium). The impurity atoms are
called acceptor atoms. The majority carriers are holes and minority carriers
are electrons.
In a semiconductor material (intrinsic or extrinsic), the law of mass action
states that
pn = constant
(10.7)
where
p
n
is the hole concentration
is the electron concentration.
For intrinsic semiconductors,
p = n = ni
(10.8)
and Equation (10.5) becomes
pn = ni2
and
(10.9)
ni is given by Equation (10.1).
The law of mass action enables us to calculate the majority and minority carrier density in an extrinsic semiconductor material. The charge neutrality
condition of a semiconductor implies that
p + ND = n + NA
© 1999 CRC Press LLC
(10.10)
where
ND
NA
p
n
is the donor concentration
is the acceptor concentration
is the hole concentration
is the electron concentration.
In an n-type semiconductor, the donor concentration is greater than the intrinsic electron concentration, i.e., N D is typically 1017 cm-3 and ni = 1.5 x
1010 cm-3 in Si at room temperature. Thus, the majority and minority concentrations are given by
nn ≅ N D
n2
p≅ i
ND
(10.11)
(10.12)
In a p-type semiconductor, the acceptor concentration
intrinsic hole concentration
centrations are given by
N A is greater than the
pi = ni . Thus, the majority and minority con-
pp ≅ N A
(10.13)
ni2
n≅
NA
(10.14)
The following example gives the minority carrier as a function of doping concentration.
Example 10.2
For an n-type semiconductor at 300oK, if the doping concentration is varied
from 1013 to 1018 atoms/cm3, determine the minority carriers in the doped
semiconductors.
Solution
From Equation (10.11) and (10.12),
© 1999 CRC Press LLC
Electron concentration =
Hole concentration =
ND
and
2
i
n
ND
where
ni = 1.5 2 x 1010 electrons/cm3
The MATLAB program is as follows:
% hole concentration in a n-type semiconductor
nd = logspace(13,18);
n = nd;
ni = 1.52e10;
ni_sq = ni*ni;
p = ni_sq./nd;
semilogx(nd,p,'b')
title('Hole concentration')
xlabel('Doping concentration, cm-3')
ylabel('Hole concentration, cm-3')
Figure 10.3 shows the hole concentration versus doping.
Figure 10.3 Hole Concentration in N-type Semiconductor (Si)
© 1999 CRC Press LLC
10.2.2
Fermi level
The Fermi level, E F , is a chemical energy of a material. It is used to describe
the energy level of the electronic state at which an electron has the probability
of 0.5 occupying that state. It is given as
mn*
1
4
E F = ( E C + EV ) − KT ln( * )
2
3
mp
(10.15)
where
E C = energy in the conduction band
EV = energy in the valence band
and k, T, mn* and mp* were defined in Section 10.1.
In an intrinsic semiconductor (Si and Ge) mn* and mp* are of the same order
of magnitude and typically, E F >> k T . Equation (10.15) simplifies to
E F = Ei ≅
1
( E + EV )
2 C
(10.16)
Equation (10.16) shows that the Fermi energy occurs near the center of the energy gap in an intrinsic semiconductor. In addition, the Fermi energy can be
thought of as the average energy of mobile carriers in a semiconductor material.
In an n-type semiconductor, there is a shift of the Fermi level towards the edge
of the conduction band. The upward shift is dependent on how much the
doped electron density has exceeded the intrinsic value. The relevant equation
is
n = ni e [
( E F − Ei )/ kT ]
where
n
ni
EF
Ei
© 1999 CRC Press LLC
is the total electron carrier density
is the intrinsic electron carrier density
is the doped Fermi level
is the intrinsic Fermi level.
(10.17)
In the case of a p-type semiconductor, there is a downward shift in the Fermi
level. The total hole density will be given by
p = ni e [
( Ei − E F )/ kT ]
(10.18)
Figure 10.4 shows the energy band diagram of intrinsic and extrinsic semiconductors.
EC
EC
EC
EF
EI = E F
EI
EI
EF
EV
EV
(a)
(b)
EV
(c )
Figure 10.4 Energy-band Diagram of (a) Intrinsic, (b) N-type, and
(c ) P-type Semiconductors.
10.2.3
Current density and mobility
Two mechanisms account for the movement of carriers in a semiconductor material: drift and diffusion. Drift current is caused by the application of an electric field, whereas diffusion current is obtained when there is a net flow of carriers from a region of high concentration to a region of low concentration. The
total drift current density in an extrinsic semiconductor material is
J = q (nµn + pµ p )Ε
where
J
n
p
µn
µp
q
© 1999 CRC Press LLC
is current density
is mobile electron density
is hole density,
is mobility of an electron
is mobility of a hole
is the electron charge
(10.19)
Ε
is the electric field.
The total conductivity is
σ = q (nµn + pµ p )Ε
(10.20)
Assuming that there is a diffusion of holes from an area of high concentration
to that of low concentration, then the current density of holes in the xdirection is
J p = − qD p
dp
dx
A/cm2
(10.21)
where
q
Dp
p
is the electronic charge
is the hole diffusion constant
is the hole concentration.
Equation (10.21) also assumes that, although the hole concentration varies
along the x-direction, it is constant in the y and z-directions. Similarly, the
electron current density, J n , for diffusion of electrons is
J n = qDn
dn
A / cm2
dx
(10.22)
where
Dn
n
For silicon,
is the electron diffusion constant
is the electron concentration.
D p = 13 cm2/s , and Dn = 200 cm2/s . The diffusion and mo-
bility constants are related, under steady-state conditions, by the Einstein relation
Dn D p kT
=
=
µn
µp
q
(10.23)
The following two examples show the effects of doping concentration on mobility and resistivity.
© 1999 CRC Press LLC
Example 10.3
From measured data, an empirical relationship between electron ( µn ) and hole
( µ p ) mobilities versus doping concentration at 300oK is given as [2]
51
. x1018 + 92 N D0.91
µn ( N D ) =
3.75x1015 + N D0.91
(10.24)
2.9 x1015 + 47.7 N A0.76
µ pn ( N A ) =
5.86 x1012 + N A0.76
(10.25)
where
N D and N A are donor and acceptor concentration per cm3,
respectively.
Plot the µn ( N D ) and µp ( N A ) for the doping concentrations from 1014 to
1020 cm-3 .
Solution
MATLAB Script
% nc - is doping concentration
%
nc = logspace(14,20);
un = (5.1e18 + 92*nc.^0.91)./(3.75e15 + nc.^0.91);
up = (2.90e15 + 47.7*nc.^0.76)./(5.86e12 + nc.^0.76);
semilogx(nc,un,'w',nc,up,'w')
text(8.0e16,1000,'Electron Mobility')
text(5.0e14,560,'Hole Mobility')
title('Mobility versus Doping')
xlabel('Doping Concentration in cm-3')
ylabel('Bulk Mobility (cm2/v.s)')
Figure 10.5 shows the plot of mobility versus doping concentration.
© 1999 CRC Press LLC
Figure 10.5 Mobility versus Doping Concentration
Example 10.4
At the temperature of 300oK, the resistivity of silicon doped by phosphorus is
given as [ 3]
ρn =
3.75x1015 + N D0.91
147
. x10 −17 N D1.91 + 815
. x10 −1 N D
(10.26)
A similar relation for silicon doped with boron is given as [ 4]
586
. x1012 + N A0.76
ρp =
7.63x10 −18 N 1A.76 + 4.64 * 10 − 4 N A
where
© 1999 CRC Press LLC
(10.27)
N D and N A are donor and acceptor concentrations, respectively.
Use MATLAB to plot the resistivity versus doping concentration (cm-3 ).
Solution
MATLAB Script
% nc is doping concentration
% rn - resistivity of n-type
% rp - resistivity of p-type
nc = logspace(14,20);
rn = (3.75e15 + nc.^0.91)./(1.47e-17*nc.^1.91 + 8.15e-1*nc);
rp = (5.86e12 + nc.^0.76)./(7.63e-18*nc.^1.76 + 4.64e-4*nc);
semilogx(nc,rn,'w',nc,rp,'w')
axis([1.0e14, 1.0e17,0,140])
title('Resistivity versus Doping')
ylabel('Resistivity (ohm-cm)')
xlabel('Doping Concentration cm-3')
text(1.1e14,12,'N-type')
text(3.0e14,50,'P-type')
Figure 10.6 shows the resistivity of N- and P-type silicon.
© 1999 CRC Press LLC
Figure 10.6 Resistivity versus Doping Concentration
10.3
10.3.1
PN JUNCTION: CONTACT POTENTIAL, JUNCTION
CURRENT
Contact potential
An ideal pn junction is obtained when a uniformly doped p-type material
abruptly changes to n-type material. This is shown in Figure 10.7.
© 1999 CRC Press LLC
P+
N
(a)
x=0
NA
X
ND
(b)
Figure 10.7 Ideal pn Junction (a) Structure, (b) Concentration of
Donors ( N D ), and acceptor ( N A ) impurities.
Practical pn junctions are formed by diffusing into an n-type semiconductor a
p-type impurity atom, or vice versa. Because the p-type semiconductor has
many free holes and the n-type semiconductor has many free electrons, there is
a strong tendency for the holes to diffuse from the p-type to the n-type semiconductors. Similarly, electrons diffuse from the n-type to the p-type material.
When holes cross the junction into the n-type material, they recombine with the
free electrons in the n-type. Similarly, when electrons cross the junction into
the p-type region, they recombine with free holes. In the junction a transition
region or depletion region is created.
In the depletion region, the free holes and electrons are many magnitudes
lower than holes in p-type material and electrons in the n-type material. As
electrons and holes recombine in the transition region, the region near the junction within the n-type semiconductor is left with a net positive charge. The region near the junction within the p-type material will be left with a net negative
charge. This is illustrated in Figure 10.8.
Because of the positive and negative fixed ions at the transition region, an electric field is established across the junction. The electric field creates a potential difference across the junction, the potential barrier. The latter is also
© 1999 CRC Press LLC
called diffusion potential or contact potential, VC . The potential barrier prevents the flow of majority carriers across the junction under equilibrium conditions.
NA = N D
-
-
P
++
++
++
N
Depletion Region
Ecp
Ec
Eip
Ef
Ein
Evp
Ev
Figure 10.8 pn Junction (a) Depletion region with Positive and
Negative Ions (b) Energy Band Diagram near a pn
Junction.
The contact potential, VC , may be obtained from the relations
nn
= e
np
qVC
kT
=
pp
pn
(10.28)
or
VC =
But, noting that
pp
n
kT
kT
ln( n ) =
ln( )
q
np
q
pn
(10.29)
ni2
ni2
pp ≅ N A , np ≅
, nn ≅ N D , pn ≅
,
NA
ND
Equation (10.29) becomes
VC =
© 1999 CRC Press LLC
N N
kT
ln( A 2 D )
q
ni
(10.30)
The contact potential can also be obtained from the band-bending diagram of
the pn junction shown in Figure 10.8. That is, from Figure 10.8
VC =
E in − E ip
(10.31)
q
or
VC = − (φ fn + φ fp )
where
(10.32)
φFN and φFP are the electron and hole Fermi potentials,
respectively. They are given as
φFN =
E F − E IN kT N D
ln
=
q
q ni
(10.33)
φFP =
E F − E IP kT N A
ln
=
q
q ni
(10.34)
and
Using Equations (10.31) to (10.34), we have
VC =
kT N A N D
ln
q ni2
(10.35)
It should be noted that Equations (10.30) and (10.35) are identical. Typically,
VC is from 0.5 to 0.8 V for the silicon pn junction. For germanium, VC is approximately 0.1 to 0.2, and that for gallium arsenide is 1.5V.
When a positive voltage VS is applied to the p-side of the junction and n-side
is grounded, holes are pushed from the p-type material into the transition region. In addition, electrons are attracted to transition region. The depletion
region decreases, and the effective contact potential is reduced. This allows
majority carriers to flow through the depletion region. Equation (10.28)
modifies to
nn
= e
np
© 1999 CRC Press LLC
q (VG −VS )
kT
=
pp
pn
(10.36)
When a negative voltage VS is applied to the p-side of a junction and the nside is grounded, the applied voltage adds directly to the contact potential.
The depletion region increases and it become more difficult for the majority
carriers to flow across the junction. The current flow is mainly due to the flow
of minority carriers. Equation (10.28) modifies to
nn
= e
np
q (VC +VS )
kT
=
pp
(10.37)
pn
Figure 10.9 shows the potential across the diode when a pn junction is
forward-biased and reversed-biased.
VS
P
N
Vc
VS = 0
VC - VS
VS > 0
VC + VS
VS < 0
Figure 10.9 PN Junction (a) with Depletion Layer and Source Connection (b) Contact Potential with No Source Voltage ( VS = 0) (c )
Junction Potential for Forward-biased pn Junction ( VS > 0) and (d)
Junction Potential for Reversed-biased pn Junction ( VS < 0)
© 1999 CRC Press LLC
The following example illustrates the effect of source voltage on the junction
potential.
Example 10.5
For a Silicon pn junction with
with
N D = 1014 cm-3 and N A = 1017 cm-3 and
ni2 = 1.04 x 1026 cm-6 at T = 300 oK,
(a)
Calculate the contact potential.
(b)
Plot the junction potential when the source voltage VS of Figure
10.9 increases from -1.0 to 0.7 V.
Solution
MATLAB Script
diary ex10_5.dat
% Junction potential versus source voltage
% using equation(10.36) contact potential is
t = 300;
na = 1.0e17;
nd = 1.0e14;
nisq = 1.04e20;
q = 1.602e-19;
k = 1.38e-23;
% calculate contact potential
vc = (k*t/q)*(log(na*nd/nisq))
vs = -1.0:0.1:0.7;
jct_pot = vc - vs;
% plot curve
plot(vs,jct_pot)
title('Junction potential vs. source voltage')
xlabel('Source voltage, V')
ylabel('Junction potential, V')
diary
© 1999 CRC Press LLC
(a) The contact potential is
vc =
0.6535
(b) Figure 10.10 shows the graph of the junction potential versus the source
voltage.
Figure 10.10 Junction Potential versus Source Voltage.
10.3.2
Junction current
The pn junction current is given as
qVS
I = I s e kT − 1
where
© 1999 CRC Press LLC
(10.38)
VS is the voltage across the pn junction [see Figure 10.9 (a)]
q is the electronic charge
T is the absolute temperature
k is Boltzmann’s constant
I S is reverse saturation current. It is given as
I S = qA(
D p pn
Lp
+
Dn n p
Ln
)
(10.39)
where
is the diode cross-sectional area
are the hole and electron diffusion lengths
A
L p , Ln
pn , n p
are the equilibrium minority carrier concentrations
D p , Dn
are the hole and electron diffusion coefficients,
respectively.
ni2
ni2
and n p ≅
, Equation (10.39) becomes
Since pn ≅
ND
NA
Dp
Dn 2
ni
I S = qA
+
L p N D Ln N A
(10.40)
The diffusion coefficient and diffusion length are related by the expression
Lp =
D pτ p
(10.41)
Ln =
Dn τ n
(10.42)
and
where
τ p , τ n are the hole minority and electron minority carrier lifetime,
respectively.
© 1999 CRC Press LLC
Equation (10.38) is the diode equation. It is applicable for forward-biased
( VS > 0 ) and reversed-biased ( VS < 0 ) pn junctions.
Using Equations (10.1) and (10.39), the reverse saturation current can
be rewritten as
I S = k 1T 3 e [
where
− E g /( kT )
]
(10.43)
k1 is a proportionality constant
Eg
− E g − Eg
dI S
2 − kT
3
e kT
= 3k1 T e
+ k 1T
dT
kT 2
Thus
1 dI S
3 1 Eg
3 1 Vg
= +
= +
I S dT T T kT T T VT
(10.44)
where
VT =
kT
q
and
Vg =
Eg
q
For silicon at room temperature,
Vg
VT
= 44.4.
Thus
V g dT
dI S
dT
= (3 + )
= 47.4
dT
VT T
T
(10.45)
At room temperature (300o K), the saturation current approximately doubles
every 5o C [5]. The following example shows how I S is affected by temperature.
© 1999 CRC Press LLC
Example 10.6
I S = 10-15 A at 25o C and assuming I S increases by
15% per oC rise in temperature, find and plot the value of I S from 25 oC to
A silicon diode has
125 oC.
Solution
From the information given above, the reverse saturation current can be expressed as
. )
I S = 10 −15 (115
MATLAB is used to find
(T − 25)
I S at various temperatures.
MATLAB Script
% Saturation current
%
t = 25:5:125;
is = 1.0e-15*(1.15).^(t-25);
plot(t,is)
title('Reverse Saturation Current vs. Temperature')
xlabel('Temperature, C')
ylabel('Current, A')
Figure 10.11 shows the effect of temperature on the reverse saturation current.
© 1999 CRC Press LLC
Figure 10.11 Reverse Saturation Current versus Temperature
10.4
DEPLETION AND DIFFUSION CAPACITANCES
10.4.1
Depletion capacitance
As mentioned previously, a pn junction is formed when a p-type material is
joined to an n-type region. During device fabrication, a p-n junction can be
formed using process such as ion-implantation diffusion or epitaxy. The doping profile at the junction can take several shapes. Two popular doping profiles are abrupt (step) junction and linearly graded junction.
In the abrupt junction, the doping of the depletion region on either side of the
metallurgical junction is a constant. This gives rise to constant charge densities on either side of the junction. This is shown in Figure 10.12.
© 1999 CRC Press LLC
VS
N
P
(a)
-WP
0
WN
qND
Charge density
(b)
-WP
-qNA
-WP
Electric field
0
x
WN
WN
x
(c)
VC - VS
Potential distribution
(d)
-WP
Figure 10.12
WN
x
PN Junction with Abrupt Junction (a) Depletion
Region (b) Charge Density (c ) Electric Field and
(d) Potential Distribution
For charge equality,
qN AWP = qN DWN
(10.46)
it can be shown [6] that the depletion width in the p-type ( WP ) and that of the
n-type material ( WN ) can be given as
WP =
© 1999 CRC Press LLC
2εN D (VC − Vs )
qN A ( N D + N A )
(10.47)
WN =
2εN A (VC − Vs )
qN D ( N D + N A )
(10.48)
where
ε
ND
NA
q
VC
VS
is the relative dielectric constant
( ε = 12ε0 for Si, and ε 0 = 8.85 x 10-12 F/m)
is donor concentration
is acceptor concentration
is electronic charge
is contact potential obtained from Equation (10.30)
is source voltage.
If the doping density on one side of the metallurgical junction is greater than
that on the other side (i.e., N A >> N D or N D >> N A ) , then the junction
properties are controlled entirely by the lightly doped side. This condition is
termed the one-sided step junction approximation. This is the practical model
for shallow junctions formed by a heavily doped diffusion into a lightly doped
region of opposite polarity [7].
In a linearly graded junction, the ionized doping charge density varies linearly
across the depletion region. The charge density passes through zero at the
metallurgical junction. Figure 10.13 shows the profile of the linearly graded
junction.
© 1999 CRC Press LLC
VS
N
P
(a)
-WP
0
WN
Charge Density -WP
WN
(b)
Electric Field
(c)
Potential
VC - VS
(d)
Figure 10.13
PN Junction with Linearly Graded Junction
(a) Depletion Region (b) Charge Density
(c ) Electric Field (d) Potential Distribution
For a linearly graded junction, the depletion width in the p-type and n-type material, on either side of the metallurgical junction, can be shown to be
12ε (VC − VS )
WN = WP =
qa
1
3
(10.49)
where
a is the slope of the graded junction impurity profile.
The contact potential is given as [6]
VC =
aW
kT
ln( N )
2 ni
q
The depletion capacitance,
C j , is due to the charge stored in the depletion re-
gion. It is generally given as
© 1999 CRC Press LLC
(10.50)
εA
WT
(10.51)
WT = WN + WP
(10.52)
Cj =
where
A
is cross-sectional area of the pn junction.
For abrupt junction, the depletion capacitance is given as
Cj = A
εqN A N D
2( N D + N A )(VC − VS )
(10.53)
For linearly graded junction, the depletion capacitance is given as
1
C j = 0.436(aq ) 3 ε
2
3
A(VC − VS )
−1
3
1
aqε 2
]3
C j = 0.436 A[
(VC − VS )
(10.54)
In general, we may express the depletion capacitance of a pn junction by
Cj =
C j0
VS
1 −
VC
m
1
1
≤m≤
3
2
where
1
for linearly graded junction and
3
1
m = for step junction
2
m=
© 1999 CRC Press LLC
(10.55)
C j 0 = zero-biased junction capacitance. It can be obtained from
Equations (10.53) and (10.54) by setting VS equal to zero.
Equations (10.53 to 10.55) are, strictly speaking, valid under the conditions of
reversed-biased VS < 0. The equations can, however, be used when VS <
VC , is the contact potential of the pn junction.
As the pn junction becomes more reversed biased ( VS < 0), the depletion ca0.2V. The positive voltage,
pacitance decreases. However, when the pn junction becomes slightly forward
biased, the capacitance increases rapidly. This is illustrated by the following
example.
Example 10.7
For a certain pn junction, with contact potential 0.065V, the junction capacitance is 4.5 pF for VS = -10 and C j is 6.5 pF for VS = -2 V.
(a) Find m and
C j 0 of Equation (10.55).
(b) Use MATLAB to plot the depletion capacitance from -30V to 0.4V.
Solution
From Equation (10.55)
C j0
V1
[1 − S ]m
VC
C j0
=
V2
[1 − S ]m
VC
C j1 =
C j2
therefore
C j1
C j2
© 1999 CRC Press LLC
V − VS 2
= C
VC − VS 1
m
C j1
log10
C j 2
m=
V − VS 2
log 10 C
VC − VS 1
(10.56)
and
C j0
V 1
= C j1 1 − S
VC
MATLAB is used to find
m
m and C j 0 . It is also used to plot the depletion ca-
pacitance.
MATLAB Script
% depletion capacitance
%
cj1 = 4.5e-12; vs1 = -10;
cj2 = 6.5e-12; vs2 = -2;
vc = 0.65;
num = cj1/cj2;
den = (vc-vs2)/(vc-vs1);
m = log10(num)/log10(den);
cj0 = cj1*(1 - (vs1/vc))^m;
vs = -30:0.2:0.4;
k = length(vs);
for i = 1:k
cj(i) = cj0/(1-(vs(i)/vc))^m;
end
plot(vs,cj,'w')
xlabel('Voltage,V')
ylabel('Capacitance,F')
title('Depletion Capacitance vs. Voltage')
axis([-30,2,1e-12,14e-12])
(a) The values of
m, C j 0 are
m =
0.02644
© 1999 CRC Press LLC
(10.57)
cj0 =
9.4246e-012
(b) Figure 10.14 shows the depletion capacitance versus the voltage across the
junction.
Figure 10.14 Depletion Capacitance of a pn Junction
10.4.2
Diffusion capacitance
When a pn junction is forward biased, holes are injected from the p-side of the
metallurgical junction into the n-type material. The holes are momentarily
stored in the n-type material before they recombine with the majority carriers
(electrons) in the n-type material. Similarly, electrons are injected into and
temporarily stored in the p-type material. The electrons then recombine with
the majority carriers (holes) in the p-type material. The diffusion capacitance,
Cd , is due to the buildup of minority carriers charge around the metallurgical
© 1999 CRC Press LLC
junction as the result of forward biasing the pn junction. Changing the forward
current or forward voltage, ∆V, will result in the change in the value of the
stored charge ∆Q, the diffusion capacitance, Cd , can be found from the general expression
Cd =
∆Q
∆V
(10.58)
It turns out that the diffusion capacitance is proportional to the forward-biased
current. That is
Cd = K d I DF
(10.59)
where
Kd
I DF
is constant at a given temperature
is forward-biased diode current.
The diffusion capacitance is usually larger than the depletion capacitance [1,
6]. Typical values of Cd ranges from 80 to 1000 pF.
A small signal model of the diode is shown in Figure 10.15.
Cd
rd
Rs
Cj
Figure 10.15 Small-signal Model of a Forward-biased pn Junction
© 1999 CRC Press LLC
In Figure 10.15,
Cd and C j are the diffusion and depletion capacitance, re-
RS is the semiconductor bulk and contact resistance. The dynamic resistance, rd , of the diode is given as
spectively.
rd =
nkT
qI DF
(10.60)
where
n
k
T
q
is
is
is
is
constant
Boltzmann’s constant
temperature in degree Kelvin
electronic charge.
When a pn junction is reversed biased,
shown in Figure 10.16.
Cd = 0. The model of the diode is
Cj
Rs
Rd
Figure 10.16 Model of a Reverse-biased pn Junction
In Figure 10.16,
C j is the depletion capacitance. The diffusion capacitance is
zero. The resistance Rd is reverse resistance of the pn junction (normally in
the mega-ohms range).
Example 10.8
A certain diode has contact potential;
VC = 0.55V, C j 0 = diffusion capaci-
tance at zero biased is 8 pF; the diffusion capacitance at 1mA is 100 pF.
Use MATLAB to plot the diffusion and depletion capacitance for forward- biased voltages from 0.0 to 0.7 V. Assume that I S = 10-14 A, n = 2.0 and stepjunction profile.
© 1999 CRC Press LLC
Solution
Using Equations (10.38) and (10.59), we write the MATLAB program to obtain the diffusion and depletion capacitance.
MATLAB Script
%
% Diffusion and depletion Capacitance
%
cd1 = 100e-12; id1 = 1.0e-3; cj0 = 8e-12; vc =0.55;
m = 0.5;
is = 1.0e-14; nd = 2.0;
k = 1.38e-23; q = 1.6e-19; T = 300;
kd = cd1/id1;
vt = k*T/q;
v = 0.0:0.05:0.55;
nv = length(v);
for i = 1:nv
id(i) = is*exp(v(i)/(nd*vt));
cd(i) = kd*id(i);
ra(i) = v(i)/vc;
cj(i) = cj0/((1 - ra(i)).^m);
end
subplot(121)
plot(v,cd)
title('Diffusion Cap.')
xlabel('Voltage, V'), ylabel('Capacitance, F')
subplot(122)
plot(v,cj)
title('Depletion Cap.')
xlabel('Voltage, V'), ylabel('Capacitance, F')
Figure 10.17 shows the depletion and diffusion capacitance of a forwardbiased pn junction.
© 1999 CRC Press LLC
(a)
(b)
Figure 10.17 (a) Depletion and (b) Diffusion Capacitance
10.5
BREAKDOWN VOLTAGES OF PN JUNCTIONS
The electric field E is related to the charge density through the Poisson’s equation
dE ( x ) ρ ( x )
=
ε S ε0
dx
where
© 1999 CRC Press LLC
εS
is the semiconductor dielectric constant
ε0
is the permittivity of free space, 8.86 * 10-14 F/cm
ρ ( x ) is the charge density.
(10.61)
For an abrupt junction with charge density shown in Figure 10.12, the charge
density
ρ ( x ) = − qN A
= qN D
− WP < x < 0
0 < x < WN
(10.62)
The maximum electric field
E max =
qN AWP qN DWN
=
ε s ε0
ε s ε0
(10.63)
Using Equation (10.47) or (10.48, Equation (10.63) becomes
2qN D N A (VC − VS )
ε S ε0 ( N A + N D )
E max =
For a linearly graded junction, the charge density,
10.13)
ρ ( x ) = ax
−
(10.64)
ρ ( x ) is given as (see Figure
W
W
<x<
2
2
(10.65)
and the maximum electric field can be shown to be
E max =
aq
W2
8ε S ε 0
(10.66)
where
a
W
is slope of charge density
is width of depletion layer and
W
= WN = WP
2
The width of the depletion region, W, can be obtained from Equation (10.49).
Equation (10.64) indicates that as the reverse voltage increases, the magnitude
of the electric field increases. The large electric field accelerates the carriers
© 1999 CRC Press LLC
crossing the junction. At a critical field, E crit , the accelerated carriers in the
depletion region have sufficient energy to create new electron-hole pairs as
they collide with other atoms. The secondary electrons can in turn create
more carriers in the depletion region. This is termed the avalanche breakdown
process. For silicon with an impurity concentration of 1016 cm-3, the critical
electric field is about 2.0x105 V/cm.
In a highly doped pn junctions, where the impurity concentration is about 1018
cm-3 , the critical electric field is about 106 V/cm. This high electric field is
able to strip electrons away from the outer orbit of the silicon atoms, thus creating hole-electron pairs in the depletion region. This mechanism of breakdown is called zener breakdown. This breakdown mechanism does not involve
any multiplication effect. Normally, when the breakdown voltage is less than
6V, the mechanism is zener breakdown process. For breakdown voltages beyond 6V, the mechanism is generally an avalanche breakdown process.
For an abrupt junction, where one side is heavily doped, the electrical properties of the junction are determined by the lightly doped side. Experimentally,
the breakdown voltage of semiconductor step junction ( n+p or p+n ) as the
function of doping concentration in the lightly doped side is given as [7]
NB
V BR = k 16
10
−0.75
(10.67)
where
k
= 25V for Ge
= 60V for Si
and
NB
is the doping concentration of lightly doped side.
The following example shows the effect of doping concentration on breakdown
voltage.
© 1999 CRC Press LLC
Example 10.9
Use MATLAB to plot the breakdown voltage versus doping concentration for
a one-sided step junction for silicon and germanium, and using doping concentration from 1014 to 1019 cm-3.
Solution
Using Equation (10.67), we calculate the breakdown voltage for various doping concentration.
MATLAB Script
%
% Breakdown voltage
%
k1 = 25;
k2 = 60;
nb = logspace(14,19);
n = length(nb);
for i = 1:n
vbr1(i) = k1*(nb(i)/1.0e16)^(-0.75); % Ge breakdown voltage
vbr2(i) = k2*(nb(i)/1.0e16)^(-0.75); % Si breakdown voltage
end
semilogx(nb,vbr1,'w', nb,vbr2,'w')
xlabel('Impurity Concentration, cm-3')
ylabel('Breakdown Voltage,V')
title('Breakdown Voltage vs. Impurity Concentration')
axis([1.0e14,1.0e17,0,2000])
text(2.0e14,270,'Ge')
text(3.0e14,1000,'Si')
Figure 10.18 shows the plot of breakdown voltage of one-sided abrupt junction.
© 1999 CRC Press LLC
Figure 10.18 Breakdown Voltage versus Impurity Concentration
REFERENCES
© 1999 CRC Press LLC
1.
Singh, J., Semiconductor Devices, McGraw Hill, 1994.
2.
Jacoboni, C., Canli, C., Ottaviani, G., and Quaranta, A.A., Review
of Some Charge Transport Properties of Silicon, Solid State Electronics, Vol. 20, pp. 77-89, 1977.
3.
Mousty, F., Ostoga, P. and Passari, L., Relationship between
Resistivity and Phosphorus Concentration in Silicon, Journal Applied
Physics, Vol. 45, pp. 4576 - 4580, 1974.
4.
Caughey, D.M. and Thomas, R.F., Carrier Mobilities in Silicon
Empirically Related to Doping and Field, Proc. IEEE, Vol. 55, pp.
2192 - 2193, 1967.
5.
Hodges, D.A. and Jackson, H.G., Analysis and Design of Digital
Integrated Circuits, McGraw Hill, 1988.
6.
Neudeck, G.W., The PN Junction Diode, Modular Series on Solid
State Devices, Vol. II, Addison-Wesley, 1983.
7.
Beadle, W.E., Tsai, J.C., and Plummer, R.D. (Editors), Quick
Reference Manual for Silicon Integrated Circuits Technology, John
Wiley and Sons, 1985.
8.
McFarlane, G.G., McLean, J.P., Quarrington, J.E., and Roberts, V.,
Fine Structure in the Absorption Edge Spectrum of Silicon, Physics
Review, Vol. 111, pp. 1245 -1254, 1958.
9.
Sze, S.M. and Gibbons, G., Avalanche Breakdown Voltages of
Abrupt and Linearly Graded p-n Junctions in Ge, Si, GaAs and GaP,
Applied Physics Letters, Vol. 8, pp. 111 - 113, 1966.
EXERCISES
10.1
In the case of silicon for temperature below 700 oK, the density of
intrinsic created carriers, ni , can be approximated as [8]
3
. * 1016 T 2 e
ni = 387
(a)
(b)
10.2
© 1999 CRC Press LLC
7 .02*103
−
T
(10.68)
Use MATLAB to plot the intrinsic carrier concentration
versus (1000/T) where T is temperature in degrees Kelvin.
Compare the above relation for intrinsic concentration with
that of Example 10.1. Plot the difference between of ni
for Equations (10.1) and (10.68).
Assuming that at 300 oK the mobile carrier concentrations of intrinsic
germanium and silicon semiconductor materials are 2.390*1013 and
1.52*1010 , respectively, use MATLAB to plot the E F − E i versus
donor concentration for Ge and Si. Assume donor concentrations
from 1010 to 1018.
10.3
For power devices with breakdown voltages above 100V and
resistivities greater than 1 ohm-cm (n-type silicon) and 3 ohm-cm (ptype silicon), the resistivity versus doping concentrations can be simplified to
ρn = 4.596 * 1015 N D−1
ρ pn = 1263
.
* 1016 N A−1
(a) Use MATLAB to plot resistivity versus doping concentration
(from 1012 to 1018 cm-3 ).
(b) Compare your results with those obtained in Example 10.4.
10.4
For Ge pn junction with
at 300 oK is 2.39*1013 ,
N A = 1018 cm-3 , N D = 1015 cm-3 and ni
(a) Calculate the contact potential.
(b) Plot the junction potential for source voltages of -1.0V to 0.3V.
10.5
For the small signal model of the forward-biased pn junction, shown
in Figure 10.15, RS = 5Ω, rd = 10 Ω, Cd = 110 pF at I DF of 1
mA. Use MATLAB to plot the equivalent input impedance (magnitude and phase) for frequencies from 104 to 1010 Hz.
10.6
Empirically, the breakdown voltage of a linearly graded junction can
be approximated as [9]
V BR
where
a −4
= k 21
10
−0.75
k = 18 V for Ge or 40 V for Si.
Use MATLAB to plot the breakdown voltage vs. impurity gradient of
Ge and Si. Use impurity gradient values from 1019 to 1024.
© 1999 CRC Press LLC
CHAPTER ELEVEN
OPERATIONAL AMPLIFIERS
The operational amplifier (Op Amp) is one of the versatile electronic circuits.
It can be used to perform the basic mathematical operations: addition, subtraction, multiplication, and division. They can also be used to do integration and
differentiation. There are several electronic circuits that use an op amp as an
integral element. Some of these circuits are amplifiers, filters, oscillators, and
flip-flops. In this chapter, the basic properties of op amps will be discussed.
The non-ideal characteristics of the op amp will be illustrated, whenever possible, with example problems solved using MATLAB.
11.1
PROPERTIES OF THE OP AMP
The op amp, from a signal point of view, is a three-terminal device: two inputs
and one output. Its symbol is shown in Figure 11.1. The inverting input is
designated by the ‘-’ sign and non-inverting input by the ‘+’ sign.
Figure 11.1 Op Amp Circuit Symbol
An ideal op amp has an equivalent circuit shown in Figure 11.2. It is a difference amplifier, with output equal to the amplified difference of the two inputs.
An ideal op amp has the following properties:
•
•
•
•
•
•
© 1999 CRC Press LLC
infinite input resistance,
zero output resistance,
zero offset voltage,
infinite frequency response and
infinite common-mode rejection ratio,
infinite open-loop gain, A.
V1
- A(V2 - V1)
V2
Figure 11.2
Equivalent Circuit of an Ideal Op Amp
A practical op amp will have large but finite open-loop gain in the range from
105 to 109. It also has a very large input resistance 106 to 1010 ohms. The output resistance might be in the range of 50 to 125 ohms. The offset voltage is
small but finite and the frequency response will deviate considerably from the
infinite frequency response. The common-mode rejection ratio is not infinite
but finite. Table 11.1 shows the properties of the general purpose 741 op
amp.
Table 11.1
Properties of 741 Op Amp
Property
Value (Typical)
Open Loop Gain
Input resistance
Output resistance
Offset voltage
Input bias current
Unity-gain bandwidth
Common-mode rejection ratio
Slew rate
2x105
2.0 M
75 Ω
1 mV
30 nA
1 MHz
95 dB
0.7 V/µV
Whenever there is a connection from the output of the op amp to the inverting
input as shown in Figure 11.3, we have a negative feedback connection
© 1999 CRC Press LLC
Z2
Z1
I1
I2
(a)
Z2
Z1
I1
I2
(b)
Figure 11.3
Negative Feedback Connections for Op Amp
(a) Inverting (b) Non-inverting configurations
With negative feedback and finite output voltage, Figure 11.2 shows that
VO = A(V2 − V1 )
(11.1)
Since the open-loop gain is very large,
(V
2
© 1999 CRC Press LLC
− V1 ) =
VO
≅0
A
(11.2)
Equation (11.2) implies that the two input voltages are also equal. This condition is termed the concept of the virtual short circuit. In addition, because of
the large input resistance of the op amp, the latter is assumed to take no current for most calculations.
11.2
INVERTING CONFIGURATION
An op amp circuit connected in an inverted closed loop configuration is shown
in Figure 11.4.
Z2
Zin
Z1
Va
Vin
A
I1
Vo
I2
Figure 11.4 Inverting Configuration of an Op Amp
Using nodal analysis at node A, we have
Va − Vin Va − VO
+
+ I1 = 0
Z1
Z2
(11.3)
From the concept of a virtual short circuit,
Va = Vb = 0
and because of the large input resistance,
plifies to
VO
Z2
=−
V IN
Z1
© 1999 CRC Press LLC
(11.4)
I 1 = 0. Thus, Equation (11.3) sim-
(11.5)
The minus sign implies that
impedance,
Z IN , is given as
Z IN =
If Z1
11.5.
VIN and V0 are out of phase by 180o. The input
VIN
= Z1
I1
(11.6)
= R1 and Z2 = R2 , we have an inverting amplifier shown in Figure
R2
Vin
R1
Vo
Figure 11.5 Inverting Amplifier
The closed-loop gain of the amplifier is
VO
R2
=−
VIN
R1
(11.7)
and the input resistance is
R1 . Normally, R2 > R1 such that V0 > V IN .
With the assumptions of very large open-loop gain and high input resistance,
the closed-loop gain of the inverting amplifier depends on the external components R1 , R2 , and is independent of the open-loop gain.
For Figure 11.4, if
Z1 = R1 and Z2 =
1
, we obtain an integrator
jwC
circuit shown in Figure 11.6. The closed-loop gain of the integrator is
VO
1
=−
VIN
jwCR1
© 1999 CRC Press LLC
(11.8)
C
Vin
IC
R1
IR
Vo
Figure 11.6 Op Amp Inverting Integrator
In the time domain
dV
V IN
= I R and I C = − C O
dt
R1
Since
(11.9)
I R = IC
VO (t ) = −
1 t
V (t )dτ + VO (0)
R1C ∫0 IN
(11.10)
The above circuit is termed the Miller integrator. The integrating time constant is CR1 . It behaves as a lowpass filter, passing low frequencies and attenuating high frequencies. However, at dc the capacitor becomes open circuited and there is no longer a negative feedback from the output to the input.
The output voltage then saturates. To provide finite closed-loop gain at dc, a
resistance R2 is connected in parallel with the capacitor. The circuit is shown
in Figure 11.7. The resistance
© 1999 CRC Press LLC
R2 is chosen such that R2 is greater than R.
R2
C
R1
Vin
Vo
Figure 11.7 Miller Integrator with Finite Closed Loop Gain at DC
For Figure 11.4, if
Z1 =
1
and Z2 = R, we obtain a differentiator cirjwC
cuit shown in Figure 11.8. From Equation (11.5), the closed-loop gain of the
differentiator is
VO
= − jwCR
VIN
(11.11)
R1
IR
C
Vin
IC
Vo
Figure 11.8 Op Amp Differentiator Circuit
In the time domain
IC = C
Since
© 1999 CRC Press LLC
dVIN
, and VO ( t ) = − I R R
dt
(11.12)
I C (t ) = I R (t )
we have
VO ( t ) = − CR
dVIN (t )
dt
(11.13)
Differentiator circuits will differentiate input signals. This implies that if an
input signal is rapidly changing, the output of the differentiator circuit will appear “ spike-like.”
The inverting configuration can be modified to produce a weighted summer.
This circuit is shown in Figure 11.9.
RF
IF
V1
V2
Vn
R1
I1
Vo
R2
I2
Rn
In
Figure 11.9 Weighted Summer Circuit
From Figure 11.9
I1 =
V1
V
V
, I 2 = 2 , ......., I n = n
R1
R2
Rn
(11.14)
also
I F = I 1 + I 2 +...... I N
(11.15)
VO = − I F RF
(11.16)
Substituting Equations (11.14) and (11.15) into Equation (11.16) we have
© 1999 CRC Press LLC
R
R
R
VO = − F V1 + F V2 +..... F V N
R2
RN
R1
(11.17)
The frequency response of Miller integrator, with finite closed-loop gain at dc,
is obtained in the following example.
Example 11.1
Vo
( jw) .
Vin
(b) If C = 1 nF and R1 = 2KΩ, plot the magnitude response for R2 equal to
For Figure 11.7, (a ) Derive the expression for the transfer function
(i) 100 KΩ, (ii) 300KΩ, and (iii) 500KΩ.
Solution
Z 2 = R2
R2
1
=
sC2 1 + sC2 R2
Z1 = R1
(11.19)
R
− 2R
Vo
1
( s) =
1 + sC2 R2
Vin
(11.20)
− 1C R
Vo
2 1
( s) =
1
Vin
s+ C R
2
2
MATLAB Script
% Frequency response of lowpass circuit
c = 1e-9; r1 = 2e3;
r2 = [100e3, 300e3, 500e3];
n1 = -1/(c*r1); d1 = 1/(c*r2(1));
num1 = [n1]; den1 = [1 d1];
w = logspace(-2,6);
h1 = freqs(num1,den1,w);
f = w/(2*pi);
© 1999 CRC Press LLC
(11.18)
(11.21)
d2 = 1/(c*r2(2)); den2 = [1 d2];
h2 = freqs(num1, den2, w);
d3 = 1/(c*r2(3)); den3 = [1 d3];
h3 = freqs(num1,den3,w);
semilogx(f,abs(h1),'w',f,abs(h2),'w',f,abs(h3),'w')
xlabel('Frequency, Hz')
ylabel('Gain')
axis([1.0e-2,1.0e6,0,260])
text(5.0e-2,35,'R2 = 100 Kilohms')
text(5.0e-2,135,'R2 = 300 Kilohms')
text(5.0e-2,235,'R2 = 500 Kilohms')
title('Integrator Response')
Figure 11.10 shows the frequency response of Figure 11.7.
Figure 11.10 Frequency Response of Miller Integrator with Finite
Closed-Loop Gain at DC
© 1999 CRC Press LLC
11.3
NON-INVERTING CONFIGURATION
An op amp connected in a non-inverting configuration is shown in Figure
11.11.
Z2
Z1
Va A
I1
Zin
Vo
Vin
Figure 11.11 Non-Inverting Configuration
Using nodal analysis at node A
Va Va − VO
+
+ I1 = 0
Z1
Z2
(11.22)
From the concept of a virtual short circuit,
VIN = Va
(11.23)
and because of the large input resistance ( i1 = 0 ), Equation (11.22) simplifies
to
VO
Z2
= 1+
VIN
Z1
(11.24)
The gain of the inverting amplifier is positive. The input impedance of the
amplifier Z IN approaches infinity, since the current that flows into the positive input of the op-amp is almost zero.
© 1999 CRC Press LLC
If Z1 = R1 and Z2 = R2 , Figure 11.10 becomes a voltage follower with gain.
This is shown in Figure 11.11.
R2
R1
Vo
Vin
Figure 11.12 Voltage Follower with Gain
The voltage gain is
VO
R2
= 1 +
VIN
R1
(11.25)
The zero, poles and the frequency response of a non-inverting configuration
are obtained in Example 11.2.
Example 11.2
For the Figure 11.13 (a) Derive the transfer function. (b) Use MATLAB to
find the poles and zeros. ( c ) Plot the magnitude and phase response, assume
that C1 = 0.1uF, C2 = 1000 0.1uF, R1 = 10KΩ, and R2 = 10 Ω.
R2
C2
Vin
Vo
V1
R1
C1
Figure 11.13 Non-inverting Configuration
© 1999 CRC Press LLC
Solution
Using voltage division
1 sC1
V1
( s) =
V IN
R1 + 1 sC1
(11.26)
From Equation (11.24)
VO
R2
( s) = 1 +
1 sC2
V1
(11.27)
Using Equations (11.26 ) and (11.27), we have
1 + sC2 R2
VO
( s) =
V IN
1 + sC1 R1
(11.28)
The above equation can be rewritten as
1
C2 R2 s +
C2 R2
VO
( s) =
V IN
1
C1 R1 s +
C1 R1
(11.29)
The MATLAB program that can be used to find the poles, zero and plot the
frequency response is as follows:
diary ex11_2.dat
% Poles and zeros, frequency response of Figure 11.13
%
%
c1 = 1e-7; c2 = 1e-3; r1 = 10e3; r2 = 10;
% poles and zeros
b1 = c2*r2;
a1 = c1*r1;
num = [b1 1];
den = [a1 1];
disp('the zero is')
z = roots(num)
© 1999 CRC Press LLC
disp('the poles are')
p = roots(den)
% the frequency response
w = logspace(-2,6);
h = freqs(num,den,w);
gain = 20*log10(abs(h));
f = w/(2*pi);
phase = angle(h)*180/pi;
subplot(211),semilogx(f,gain,'w');
xlabel('Frequency, Hz')
ylabel('Gain, dB')
axis([1.0e-2,1.0e6,0,22])
text(2.0e-2,15,'Magnitude Response')
subplot(212),semilogx(f,phase,'w')
xlabel('Frequency, Hz')
ylabel('Phase')
axis([1.0e-2,1.0e6,0,75])
text(2.0e-2,60,'Phase Response')
diary
The results are:
the zero is
z=
-100
the pole is
p=
-1000
The magnitude and phase plots are shown in Figure 11.14
© 1999 CRC Press LLC
Figure 11.14 Frequency Response of Figure 11.13
11.4
EFFECT OF FINITE OPEN-LOOP GAIN
For the inverting amplifier shown in Figure 11.15, if we assume a finite openloop gain A, the output voltage V0 can be expressed as
VO = A(V2 − V1 )
Since
V2 = 0 ,
V1 = −
© 1999 CRC Press LLC
VO
A
(11.30)
R2
IR2
R1
V1
Vin
Vo
V2
IR1
A (V2-V1)
Figure 11.15
Inverter with Finite Open-loop Gain
Because the op amp has a very high input resistance,
I R1 = I R 2
But
I R1 =
VIN − V1 VIN − V0 A
=
R1
R1
i 1 = 0, we have
(11.31)
(11.32)
Also
VO = V1 − I R 2 R2
(11.33)
Using Equations (11.30), (11.31) and (11.32), Equation (11.33) becomes
VO = −
VO R2
−
(V + VO A)
A R1 IN
(11.34)
Simplifying Equation (11.34), we get
VO
R2 R1
=−
VIN
1 + (1 + R2 R1 ) A
© 1999 CRC Press LLC
(11.35)
It should be noted that as the open-loop gain approaches infinity, the closedloop gain becomes
VO
R2
≅−
VIN
R1
The above expression is identical to Equation (11.7).
In addition, from
Equation (11.30) , the voltage V1 goes to zero as the open-loop gain goes to
infinity. Furthermore, to minimize the dependence of the closed-loop gain on
the value of the open-loop gain, A, we should make
R2
1 + << A
R1
(11.36)
This is illustrated by the following example.
Example 11.3
In Figure 11.15, R1 = 500 Ω, and R2 = 50 KΩ. Plot the closed-loop gain as
the open-loop gain increases from 102 to 108 .
Solution
% Effect of finite open-loop gain
%
a = logspace(2,8);
r1 = 500; r2 = 50e3; r21 = r2/r1;
g = [];
n = length(a);
for i = 1:n
g(i) = r21/(1+(1+r21)/a(i));
end
semilogx(a,g,'w')
xlabel('Open loop gain')
ylabel('Closed loop gain')
title('Effect of Finite Open Loop Gain')
axis([1.0e2,1.0e8,40,110])
© 1999 CRC Press LLC
Figure 11.16 shows the characteristics of the closed-loop gain as a function of
the open-loop gain.
Figure 11.16
Closed-Loop Gain versus Open-Loop Gain
For the voltage follower with gain shown in Figure 11.12, it can be shown that
the closed-loop gain of the amplifier with finite open-loop gain is
(1 + R2 R1 )
VO
=−
VIN
1 + (1 + R2 R1 ) A
© 1999 CRC Press LLC
(11.37)
11.5
FREQUENCY RESPONSE OF OP AMPS
The simplified block diagram of the internal structure of the operational amplifier is shown in Figure 11.17.
V1
Voltage
amplifier
and level
shifter
Difference
amplifier
V2
output
stage
amplifier
Vo
Figure 11.17 Internal Structure of Operational Amplifier
Each of the individual sections of the operational amplifier contains a lowpass
RC section, with its corner (pole) frequency. Thus, an op amp will have an
open-loop gain with frequency that can be expressed as
A( s) =
AO
(1 + s w )(1 + s w )(1 + s w )
1
where
2
(11.38)
3
w1 < w2 < w3
AO = gain at dc
For most operational amplifiers,
and
w1 is very small (approx. 20π radians /s)
w2 might be in the range of 2 to 6 mega-radians/s.
Example 11.4
The constituent parts of an operational amplifier have the following internal
characteristics: the pole of the difference amplifier is at 200 Hz and the gain is
- 500. The pole of the voltage amplifier and level shifter is 400 KHz and has a
gain of 360. The pole of the output stage is 800KHz and the gain is 0.92.
Sketch the magnitude response of the operational amplifier open-loop gain.
© 1999 CRC Press LLC
Solution
The lowpass filter response can be expressed as
Crstage
VO
jw) = −
(
1 + jf f p
VIN
(11.39)
C
VO
( s) = rstage
1 + s wp
VIN
(11.40)
or
The transfer function of the amplifier is given as
A( s) =
− 500
0.92
360
(1 + s 400π ) (1 + s 8π 10 ) (1 + s 16. π 10 )
5
6
(11.41)
The above expression simplifies to
A( s) =
2.62 x10 21
( s + 400π )(s + 8π 105 )(s + 16
. π 10 6 )
MATLAB script
% Frequency response of op amp
% poles are
p1 = 400*pi; p2 = 8e5*pi; p3 = 1.6e6*pi;
p = [p1 p2 p3];
% zeros
z = [0];
const = 2.62e21;
% convert to poles and zeros and
% find the frequency response
a3 = 1;
a2 = p1 + p2 + p3;
a1 = p1*p2 + p1*p3 + p2*p3;
a0 = p1*p2*p3;
den = [a3 a2 a1 a0];
num = [const];
w = logspace(1,8);
© 1999 CRC Press LLC
(11.42)
h = freqs(num,den,w);
f = w/(2*pi);
g_db = 20*log10(abs(h));
% plot the magnitude response
semilogx(f,g_db)
title('Magnitude response')
xlabel('Frequency, Hz')
ylabel('Gain, dB')
The frequency response of the operational amplifier is shown in Figure 11.18.
Figure 11.18 Open-Loop Gain Characteristics of an Op Amp
For an internally compensated op amp, there is a capacitor included on the IC
chip. This causes the op amp to have a single pole lowpass response. The
process of making one pole dominant in the open-loop gain characteristics is
called frequency compensation, and the latter is done to ensure the stability of
the op amp. For an internally compensated op amp, the open-loop gain A( s)
can be written as
© 1999 CRC Press LLC
A( s) =
AO
(1 + s w )
(11.43)
b
where
A0
wb
is dc open-loop gain
is break frequency.
For the 741 op amp,
quencies
A0 = 105 and wb = 20 π radians/s. At physical fre-
s = jw, Equation (11.43) becomes
A( jw) =
AO
(1 + jw w )
(11.44)
b
For frequencies
w > wb , Equation (11.44) can be approximated by
A( jw) =
AO wb
jw
The unity gain bandwidth,
is given as
(11.45)
wt (the frequency at which the gain goes to unity),
wt = AO wb
(11.46)
For the inverting amplifier shown in Figure 11.5, if we substitute Equation
(11.43) into Equation (11.35), we get a closed-loop gain
VO
( s) = −
V IN
R2 R1
1 + (1 + R2 R1 ) Ao +
s
(11.47)
wt (1 + R2 R1 )
In the case of non-inverting amplifier shown in Figure 11.12, if we substitute
Equation (11.43) into Equation (11.37), we get the closed-loop gain expression
© 1999 CRC Press LLC
VO
( s) =
V IN
1 + R2 R1
1 + (1 + R2 R1 ) Ao +
s
(11.48)
wt (1 + R2 R1 )
From Equations (11.47) and (11.48), it can be seen that the break frequency for
the inverting and non-inverting amplifiers is given by the expression
w3dB =
wt
1 + R2 R1
(11.49)
The following example illustrates the effect of the ratio
R2
on the frequency
R1
response of op amp circuits.
Example 11.5
10 7 , the unity gain bandwidth of
An op amp has an open-loop dc gain of
108 Hz.
For an op amp connected in an inverting configuration (Figure
11.5), plot the magnitude response of the closed-loop gain.
if
R2
= 100 , 600, 1100
R1
Solution
Equation (11.47) can be written as
wt R2
R
R1 (1 + 2 R )
Vo
1
( s) =
wt
wt
V IN
+
s+
A0 (1 + R2 )
R
1
MATLAB script
% Inverter closed-loop gain versus frequency
w = logspace(-2,10); f = w/(2*pi);
r12 = [100 600 1100];
© 1999 CRC Press LLC
(11.50)
a =[]; b = []; num = []; den = []; h = [];
for i = 1:3
a(i) = 2*pi*1.0e8*r12(i)/(1+r12(i));
b(i) = 2*pi*1.0e8*((1/(1+r12(i))) + 1.0e-7);
num = [a(i)];
den = [1 b(i)];
h(i,:) = freqs(num,den,w);
end
semilogx(f,abs(h(1,:)),'w',f,abs(h(2,:)),'w',f,abs(h(3,:)),'w')
title('Op Amp Frequency Characteristics')
xlabel('Frequency, Hz')
ylabel('Gain')
axis([1.0e-2,1.0e10,0,1200])
text(1.5e-2, 150, 'Resistance ratio of 100')
text(1.5e-2, 650, 'Resistance ratio of 600')
text(1.50e-2, 1050, 'Resistance ratio of 1100')
Figure 11.19 shows the plots obtained from the MATLAB program.
Figure 11.19 Frequency Response of an Op Amp Inverter with
Different Closed Loop Gain
© 1999 CRC Press LLC
11.6
SLEW RATE AND FULL-POWER BANDWIDTH
Slew rate (SR) is a measure of the maximum possible rate of change of the output voltage of an op amp. Mathematically, it is defined as
SR =
dVO
dt
(11.51)
max
The slew rate is often specified on the op amp data sheets in V/µs. Poor op
amps might have slew rates around 1V/µs and good ones might have slew rates
up to 1000 V/µs are available, but the good ones are relatively expensive.
Slew rate is important when an output signal must follow a large input signal
that is rapidly changing. If the slew rate is lower than the rate of change of the
input signal, then the output voltage will be distorted. The output voltage will
become triangular, and attenuated. However, if the slew rate is higher than the
rate of change of the input signal, no distortion occurs and input and output of
the op amp circuit will have similar wave shapes.
As mentioned in the Section (11.5), frequency compensated op amp has an internal capacitance that is used to produce a dominant pole. In addition, the op
amp has a limited output current capability, due to the saturation of the input
stage. If we designate I max as the maximum possible current that is available
to charge the internal capacitance of an op amp, the charge on the frequencycompensation capacitor is
CdV = Idt
Thus, the highest possible rate of change of the output voltage is
SR =
dVO
dt
=
max
I max
C
(11.52)
For a sinusoidal input signal given by
vi ( t ) = Vm sin wt
The rate of change of the input signal is
© 1999 CRC Press LLC
(11.53)
dvi (t )
= wVm cos wt
dt
(11.54)
Assuming that the input signal is applied to a unity gain follower, then the output rate of change
dVO dvi (t )
=
= wVm cos wt
dt
dt
(11.55)
The maximum value of the rate of change of the output voltage occurs when
cos( wt ) = 1, i.e., wt = 0, 2π , 4π . ..., the slew rate
SR =
dVO
dt
= wVm
(11.56)
max
Equation (11.56) can be used to define full-power bandwidth. The latter is the
frequency at which a sinusoidal rated output signal begins to show distortion
due to slew rate limiting. Thus
wmVo ,rated = SR
(11.57)
SR
2π ,Vo ,rated
(11.58)
Thus
fm =
The full-power bandwidth can be traded for output rated voltage, thus, if the
output rated voltage is reduced, the full-power bandwidth increases. The following example illustrates the relationship between the rated output voltage
and the full-power bandwidth.
Example 11.6
The LM 741 op amp has a slew rate of 0.5 V/µs. Plot the full-power bandwidth versus the rated output voltage if the latter varies from ± 1 to ± 10 V.
Solution
% Slew rate and full-power bandwidth
sr = 0.5e6;
© 1999 CRC Press LLC
v0 = 1.0:10;
fm = sr./(2*pi*v0);
plot(v0,fm)
title('Full-power Bandwidth vs. Rated Output Voltage')
xlabel('Rated output voltage, V')
ylabel('Bandwidth, Hz')
Figure 11.20 shows the plot for Example 11.6.
Figure 11.20 Rated Output Voltage versus Full-power Bandwidth
11.7
COMMON-MODE REJECTION
For practical op amps, when two inputs are tied together and a signal applied
to the two inputs, the output will be nonzero. This is illustrated in Figure
11.21a, where the
© 1999 CRC Press LLC
Vo
Vi,cm
(a)
+
Vo
Vid
-
(b)
Figure 11.21
common-mode gain,
Circuits Showing the Definitions of (a) Commonmode Gain and (b) Differential-mode Gain
Acm , is defined as
vo
vi ,cm
Acm =
The differential-mode gain,
Ad =
(11.59)
Ad , is defined as
vo
vid
(11.60)
For an op amp with arbitrary input voltages,
the differential input signal,
V1 and V2 (see Figure 11.21b),
v id , is
vid = V2 − V1
(11.61)
and the common mode input voltage is the average of the two input signals,
Vi ,cm =
V2 + V1
2
The output of the op amp can be expressed as
© 1999 CRC Press LLC
(11.62)
VO = Ad vid + Acm vi ,cm
(11.63)
The common-mode rejection ratio (CMRR) is defined as
CMRR =
Ad
Acm
(11.64)
The CMRR represents the op amp’s ability to reject signals that are common to
the two inputs of an op amp. Typical values of CMRR range from 80 to 120
dB. CMRR decreases as frequency increases.
For an inverting amplifier as shown in Figure 11.5, because the non-inverting
input is grounded, the inverting input will also be approximately 0 V due to
the virtual short circuit that exists in the amplifier. Thus, the common-mode
input voltage is approximately zero and Equation (11.63) becomes
VO ≅ Ad Vid
(11.65)
The finite CMRR does not affect the operation of the inverting amplifier.
A method normally used to take into account the effect of finite CMRR in calculating the closed-loop gain is as follows: The contribution of the output
voltage due to the common-mode input is AcmVi ,cm . This output voltage contribution can be obtained if a differential input signal,
input of an op amp with zero common-mode gain.
Thus
Verror Ad = AcmVi ,cm
Verror =
AcmVi ,cm
Ad
=
Verror , is applied to the
(11.66)
Vi ,cm
CMRR
(11.67)
Figure 11.22 shows how to use the above technique to analyze a non-inverting
amplifier with a finite CMRR.
© 1999 CRC Press LLC
R2
R1
Vo
Vi
Finite CMRR
(a)
R2
R1
Vo
Infinite CMRR
Verror
Vi
(b)
Figure 11.22 Non-inverting Amplifier (a) Finite CMRR
( b) Infinite CMRR
From Figure 11.22b, the output voltage is given as
VO = Vi (1 + R2 R1 ) +
Vi
(1 + R2 R1 )
CMRR
(11.68)
The following example illustrates the effect of a finite CMRR on the closedloop gain of a non-inverting amplifier.
© 1999 CRC Press LLC
Example 11.7
For the amplifier shown in Figure 11.22, if R2 = 50KΩ and R1 = 1KΩ, plot
the closed-loop gain versus CMRR for the following values of the latter:
10 4 , 105 , 10 6 , 10 7 , 108 and 10 9 .
Solution
MATLAB Script
% Non-inverting amplifier with finite CMRR
r2 = 50e3; r1 = 1.0e3; rr = r2/r1;
cmrr = logspace(4,9,6); gain = (1+rr)*(1+1./cmrr);
semilogx(cmrr,gain,'wo')
xlabel('Common-mode Rejection Ratio')
ylabel('Closed Loop Gain')
title('Gain versus CMRR')
axis([1.0e3,1.0e10,50.998, 51.008])
Figure 11.23 shows the effect of CMRR on the closed loop of a non-inverting
amplifier.
Figure 11.23
© 1999 CRC Press LLC
Effect of finite CMRR on the Gain of a Noninverting Amplifier
SELECTED BIBLIOGRAPHY
1.
Schilling, D.L. and Belove, C., Electronic Circuits - Discrete and
Integrated, 3rd Edition, McGraw Hill, 1989.
2.
Wait, J.V., Huelsman, L.P., and Korn, G.A., Introduction to
Operational Amplifiers - Theory and Applications, 2nd Edition,
McGraw Hill, 1992.
3.
Sedra, A.S. and Smith, K.C., Microelectronics Circuits, 4th Edition,
Oxford University Press, 1997.
4.
Ferris, C.D., Elements of Electronic Design, West Publishing, 1995.
5.
Irvine, R.G., Operational Amplifiers - Characteristics and
Applications, Prentice Hall, 1981.
6.
Ghausi, M.S., Electronic Devices and Circuits: Discrete and
Integrated, HRW, 1985.
EXERCISES
11.1
For the circuit shown in Figure P11.1, (a) derive the transfer function
VO
( s) . (b) If R1 = 1KΩ, obtain the magnitude response.
V IN
20 kilohms
Vin
R1
1nF
Vo
Figure P11.1 An Op Amp Filter
11.2
© 1999 CRC Press LLC
For Figure 11.12, if the open-loop gain is finite, (a) show that the
closed-loop gain is given by the expression shown in Equation
(11.37). (b) If R2 = 100K and R1 = 0.5K, plot the percentage error
in the magnitude of the closed-loop gain for open-loop gains of
10 2 , 10 4 , 10 6 and 10 8 .
11.3
Find the poles and zeros of the circuit shown in Figure P11.3. Use
MATLAB to plot the magnitude response. The resistance values are
in kilohms.
10
1 nF
1 nF
Vo
Vin
1
Figure P11.3 An Op Amp Circuit
11.4
For the amplifier shown in Figure 11.12, if the open-loop gain is 106,
R2 = 24K, and R1 = 1K, plot the frequency response for a unity gain
bandwidth of
11.5
10 6 , 10 7 , and 10 8 Hz.
For the inverting amplifier, shown in Figure 11.5, plot the 3-dB
frequency versus resistance ratio
R2
for the following values of the
R1
resistance ratio: 10, 100, 1000, 10,000 and 100,000. Assume that
AO = 10 6 and f t = 10 7 Hz.
11.6
For the inverting amplifier, shown in Figure 11.5, plot the closed
loop gain versus resistance ratio
gain,
© 1999 CRC Press LLC
R2
R1
for the following open-loop
AO : 103, 105 and 107. Assume a unity gain bandwidth of
f t = 10 7 Hz and resistance ratio,
R2
has the following values: 10,
R1
100, 1000, 10,000 and 100,000.
11.7
An op amp with a slew rate of 1 V/µs is connected in the unity gain
follower configuration. A square wave of zero dc voltage and a peak
voltage of 1 V and a frequency of 100 KHz is connected to the input
of the unity gain follower. Write a MATLAB program to plot the
output voltage of the amplifier.
11.8
For the non-inverting amplifier, if
Ricm = 400 MΩ, Rid = 50 MΩ,
R1 = 2KΩ and R2 = 30KΩ, plot the input resistance versus the dc
open-loop gain A0 . Assume the following values of the open-loop
3
5
7
9
gain: 10 , 10 , 10 and 10 .
© 1999 CRC Press LLC
CHAPTER TWELVE
TRANSISTOR CIRCUITS
In this chapter, MATLAB will be used to solve problems involving metaloxide semiconductor field effect and bipolar junction transistors. The general
topics to be discussed in this chapter are dc model of BJT and MOSFET,
biasing of discrete and integrated circuits, and frequency response of
amplifiers.
12.1 BIPOLAR JUNCTION TRANSISTORS
Bipolar junction transistor (BJT) consists of two pn junctions connected backto-back. The operation of the BJT depends on the flow of both majority and
minority carriers. There are two types of BJT: npn and pnp transistors. The
electronic symbols of the two types of transistors are shown in Figure 12.1.
C
IC
B
IC
B
IB
IB
E
IE
(a)
Figure 12.1 (a) NPN transistor
C
IE
(b)
(b) PNP Transistor
The dc behavior of the BJT can be described by the Ebers-Moll Model. The
equations for the model are
© 1999 CRC Press LLC
V
I F = I ES exp BE − 1
VT
(12.1)
V
I R = I CS exp BC − 1
VT
(12.2)
and
and
IC = αF I F − I R
(12.3)
IE = −I F + αR IR
(12.4)
I B = (1 − α F )I F + (1 − α R )I R
(12.5)
where
I ES and I CS are the base-emitter and base-collector saturation
currents, respectively
α R is large signal reverse current gain of a common-base
configuration
α F is large signal forward current gain of the common-base
configuration.
and
VT =
kT
q
(12.6)
where
k
T
q
is the Boltzmann’s constant ( k = 1.381 x 10-23 V.C/ o K ),
is the absolute temperature in degrees Kelvin, and
is the charge of an electron (q = 1.602 x 10-19 C).
The forward and reverse current gains are related by the expression
α R I CS = α F I ES = I S
(12.7)
where
IS
is the BJT transport saturation current.
α R and α F are influenced by impurity concentrations and
junction depths. The saturation current, I S , can be expressed as
The parameters
© 1999 CRC Press LLC
IS = JS A
(12.8)
where
A
JS
is the area of the emitter and
is the transport saturation current density, and it can be
further expressed as
qDn ni2
JS =
QB
(12.9)
where
Dn
ni
QB
is the average effective electron diffusion constant
is the intrinsic carrier concentration in silicon ( ni = 1.45 x
1010 atoms / cm3 at 300o K)
is the number of doping atoms in the base per unit area.
The dc equivalent circuit of the BJT is based upon the Ebers-Moll model.
The model is shown in Figure 12.2. The current sources α R I R indicate the
interaction between the base-emitter and base-collector junctions due to the
narrow base region.
In the case of a pnp transistor, the directions of the diodes in Figure 12.2 are
reversed. In addition, the voltage polarities of Equations (12.1) and (12.2) are
reversed. The resulting Ebers-Moll equations for pnp transistors are
© 1999 CRC Press LLC
V
V
I E = I ES exp EB − 1 − α R I CS exp CB − 1
VT
VT
(12.10)
V
V
I C = −α F I ES exp EB − 1 + I CS exp CB − 1
VT
VT
(12.11)
IC
+
VBC
-
IB
+
VBE
-
α RIF
IR
IF
α RIF
IE
Figure 12.2 Ebers-Moll Static Model for an NPN transistor
(Injection Version)
The voltages at the base-emitter and base-collector junctions will define the
regions of operation. The four regions of operations are forward-active,
reverse-active, saturation and cut-off. Figure 12.3 shows the regions of
operation based on the polarities of the base-emitter and base collector
junctions.
Forward-Active Region
The forward-active region corresponds to forward biasing the emitter-base
junction and reverse biasing the base-collector junction. It is the normal
operational region of transistors employed for amplifications. If V BE > 0.5 V
and
as
V BC < 0.3V, then equations (12.1) to (12.4) and (12.6) can be rewritten
V
I C = I S exp BE
VT
© 1999 CRC Press LLC
(12.12)
IE = −
V
IS
exp BE
αF
VT
(12.13)
From Figure 12.1,
I B = −( I C + I E )
(12.14)
Substituting Equations (12.12) and (12.13) into (12.14), we have
IB = IS
=
(1 − α )
F
αF
V
exp BE
VT
V
IS
exp BE
βF
VT
(12.15)
(12.16)
where
βF = large signal forward current gain of common-emitter
configuration
βF =
αF
1 − αF
(12.17)
From Equations (12.12) and (12.16), we have
I C = βF I B
(12.18)
We can also define, βR , the large signal reverse current gain of the commonemitter configuration as
βR =
© 1999 CRC Press LLC
αR
1 − αR
(12.19)
VBC
reverse-active
forward bias
saturation
VBE
reverse bias
reverse bias
forward bias
cut-off
forward-active
Figure 12.3 Regions of Operation for a BJT as Defined by the Bias
of V BE and V BC
Reverse-Active Region
The reverse-active region corresponds to reverse biasing the emitter-base
junction and forward biasing the base-collector junction. The Ebers-Moll
model in the reverse-active region (VBC > 0.5V and VBE < 0.3V) simplifies to
V
I E = I S BC
VT
IB =
Thus,
V
IS
exp BC
βR
VT
I E = βR I B
The reverse-active region is seldom used.
© 1999 CRC Press LLC
(12.20)
(12.21)
(12.22)
Saturation and Cut-off Regions
The saturation region corresponds to forward biasing both base-emitter and
base-collector junctions. A switching transistor will be in the saturation region
when the device is in the conducting or “ON” state.
The cut-off region corresponds to reverse biasing the base-emitter and basecollector junctions. The collector and base currents are very small compared
to those that flow when transistors are in the active-forward and saturation
regions.
In most applications, it is adequate to assume that
I C = I B = I E = 0 when a BJT is in the cut-off region. A switching
transistor will be in the cut-off region when the device is not conducting or in
the “OFF” state.
Example 12.1
βF = 120, βR = 0.3
Assume that a BJT has an emitter area of 5.0 mil2,
transport current density,
I E versus V BE for V BC
µA / mil and T = 300oK. Plot
= -1V. Assume 0 < V BE < 0.7 V.
J S = 2 * 10
−10
2
Solution
From Equations (12.1), (12.2) and (12.4), we can write the following
MATLAB program.
MATLAB Script
%Input characteristics of a BJT
diary ex12_1.dat
diary on
k=1.381e-23; temp=300; q=1.602e-19;
cur_den=2e-10; area=5.0; beta_f=120; beta_r=0.3;
vt=k*temp/q; is=cur_den*area;
alpha_f=beta_f/(1+beta_f);
alpha_r = beta_r/(1+beta_r);
ies=is/alpha_f;
vbe=0.3:0.01:0.65;
ics=is/alpha_r;
m=length(vbe)
for i = 1:m
ifr(i) = ies*exp((vbe(i)/vt)-1);
© 1999 CRC Press LLC
ir1(i) = ics*exp((-1.0/vt)-1);
ie1(i) = abs(-ifr(i) + alpha_r*ir1(i));
end
plot(vbe,ie1)
title('Input characteristics')
xlabel('Base-emitter voltage, V')
ylabel('Emitter current, A')
Figure 12.4 shows the input characteristics.
Figure 12.4 Input Characteristics of a Bipolar Junction Transistor
Experimental studies indicate that the collector current of the BJT in the
forward-active region increases linearly with the voltage between the collectoremitter VCE. Equation 12.12 can be modified as
V
I C ≅ I S exp BE
VT
VCE
1 +
V AF
where
V AF
© 1999 CRC Press LLC
is a constant dependent on the fabrication process.
(12.23)
Example 12.2
α F = 0.98, α R = 0.35,
V AF = 250 V and transport current density is 2.0 x 10 −9 µA / mil 2 . Use
MATLAB to plot the output characteristic for V BE = 0.65 V. Neglect the
effect of VAF on the output current I C . Assume a temperature of 300 oK.
For an npn transistor with emitter area of 5.5 mil2,
Solution
MATLAB Script
%output characteristic of an npn transistor
%
diary ex12_2.dat
k=1.381e-23; temp=300; q=1.602e-19;
cur_den=2.0e-15; area=5.5; alpha_f=0.98;
alpha_r=0.35; vt=k*temp/q; is=cur_den*area;
ies=is/alpha_f; ics=is/alpha_r;
vbe= [0.65];
vce=[0 0.07 0.1 0.2 0.3 0.4 0.5 0.6 0.7 1 2 4 6];
n=length(vbe);
m=length(vce);
for i=1:n
for j=1:m
ifr(i,j)= ies*exp((vbe(i)/vt) - 1);
vbc(j) = vbe(i) - vce(j);
ir(i,j) = ics*exp((vbc(j)/vt) - 1);
ic(i,j) = alpha_f*ifr(i,j) - ir(i,j);
end
end
ic1 = ic(1,:);
plot(vce, ic1,'w')
title('Output Characteristic')
xlabel('Collector-emitter Voltage, V')
ylabel('Collector current, A')
text(3,3.1e-4, 'Vbe = 0.65 V')
axis([0,6,0,4e-4])
Figure 12.5 shows the output characteristic.
© 1999 CRC Press LLC
Figure 12.5 Output Characteristic on an NPN Transistor
12.2
12.2.1
BIASING BJT DISCRETE CIRCUITS
Self-bias circuit
One of the most frequently used biasing circuits for discrete transistor circuits
is the self-bias of the emitter-bias circuit shown in Figure 12.6.
VCC
RBI
RC
RB2
RE
(a)
© 1999 CRC Press LLC
CE
VCC
IC
RC
+
IB
VCE
RBB
VBB
-
RE
+
IE
(b)
Figure 12.6 (a) Self-Bias Circuit (b) DC Equivalent Circuit of (a)
The emitter resistance,
and
RE , provides stabilization of the bias point. If VBB
RB are the Thevenin equivalent parameters for the base bias circuit, then
V BB =
VCC RB 2
R B1 + R B 2
R B = R B1 R B 2
(12.24)
(12.25)
Using Kirchoff’s Voltage Law for the base circuit, we have
VBB = I B RB + VBE + I E RE
(12.26)
Using Equation (12.18) and Figure 12.6b, we have
I E = I B + I C = I B + βF I B = (βF + 1)I B
Substituting Equations (12.18) and (12.27) into (12.26), we have
© 1999 CRC Press LLC
(12.27)
IB =
VBB − V BE
RB + (βF + 1)RE
(12.28)
or
IC =
VBB − V BE
R B (βF + 1)
RE
+
βF
βF
(12.29)
Applying KVL at the output loop of Figure 12.6b gives
VCE = VCC − I C RC − I E RE
(12.30)
R
= VCC − I C RC + E α
F
12.2.2
(12.31)
Bias stability
Equation (12.30) gives the parameters that influence the bias current
I C . The
VBB depends on the supply voltage VCC . In some cases, VCC would
vary with I C , but by using a stabilized voltage supply we can ignore the
changes in VCC , and hence VBB . The changes in the resistances R BB and
RE are negligible. There is a variation of βF with respect to changes in I C .
A typical plot of βF versus I C is shown in Figure 12.7.
voltage
1
Bf
________
Bfmax
0.5
IC
Figure 12.7 Normalized plot of
Current
© 1999 CRC Press LLC
βF as a Function of Collector
Temperature changes cause two transistor parameters to change. These are (1)
base-emitter voltage ( V BE ) and (2) collector leakage current between the base
and collector ( I CBO ). The variation on V BE with temperature is similar to
the changes of the pn junction diode voltage with temperature. For silicon
transistors, the voltage V BE varies almost linearly with temperature as
∆VBE ≅ −2(T2 − T1 )
mV
(12.32)
where
T1 and T2 are in degrees Celsius.
I CBO , approximately doubles every 10o
temperature rise. As discussed in Section 9.1, if I CBO1 is the reverse leakage
The collector-to-base leakage current,
current at room temperature (25 oC), then
I CBO 2 = 2
and
T2 −25O C / 10
I CBO1
∆I CBO = I CBO 2 − I CBO1 = I
T2 −25O C /10
= I CBO 2
− 1
Since the variations in
(12.33)
I CBO and VBE are temperature dependent, but changes
VCC and βF are due to factors other than temperature, the information
about the changes in VCC and βF must be specified.
in
From the above discussion, the collector current is a function of four variables:
VBE , I CBO , βF , VCC . The change in collector current can be obtained using
partial derivatives. For small parameter changes, a change in collector current
is given as
∆I C =
© 1999 CRC Press LLC
∂I
∂I C
∂I C
∂I
∆VBE +
∆I CBO + C ∆βF + C ∆VCC
∂VCC
∂VBE
∂VCBO
∂βF
(12.34)
The stability factors can be defined for the four variables as
Sβ =
∂I C
∆I
≅ C
∂βF ∆βF
Sv =
∂I C
∆I C
≅
∂VBE ∆V BE
SI =
∂I C
∆I C
≅
∂I CBO ∆I CBO
and
SVCC =
∂I C
∆I C
≅
∂VCC ∆VCC
(12.35)
Using the stability factors, Equation (12.34) becomes
∆I C = SV ∆VBE + S β ∆βF + S I ∆I CBO + SVCC ∆VCC
(12.36)
From Equation (12.30),
SV =
dI C
=−
RB
dV BE
1
βF + 1
βF + R E
βF
(12.37)
From Equation (12.31),
IC =
VCC − VCE
R
RC + E α
F
Thus, the stability factor
(12.38)
SVCC is given as
SVCC =
dI C
1
=
dVCC RC + RE α F
(12.39)
To obtain the stability factor S I , an expression for I C involving I CBO needs
to be derived. The derivation is assisted by referring to Figure 12.8.
© 1999 CRC Press LLC
ICBO
IC
Ic '
IB
IE
Figure 12.8 Current in Transistor including
I CBO
The current
and
I C = I C' + I CBO
(12.40)
I C' = βF (I B + I CBO )
(12.41)
From Equations (12.40) and (12.41), we have
I C = βF I B + (βF + 1)I CBO
Assuming that
(12.42)
βF + 1 ≅ βF , then
I C = βF I B + βF I CBO
(12.43)
IC
− I CBO
βF
(12.44)
so
IB =
The loop equation of the base-emitter circuit of Figure 12.6(b) gives
VBB − VBE = I B R BB + RE ( I B + I C )
= I B ( RBB + RE ) + RE I C
© 1999 CRC Press LLC
(12.45)
Assuming that βF
we get
+ 1 ≅ βF and substituting Equation (12.44) into (12.45),
I
VBB − VBE = ( RBB + RE ) C − I CBO + I C R E
βF
Solving for I C , we have
IC =
V BB − VBE + ( RBB + RE )I CBO
(R
+ RE )
BB
(12.46)
(12.47)
βF + R E
Taking the partial derivative,
SI =
∂I C
RBB + RE
=
∂I CBO ( RBB + R E )
βF + RE
The stability factor involving
(12.48)
βF and S β can also be found by taking the
partial derivative of Equation (12.47). Thus,
Sβ =
[
∂I C ( R B + RE ) VBB − V BE + ( R B + RE ) I CBO
=
∂β
( RB + RE + βRE )2
]
(12.49)
The following example shows the use of MATLAB for finding the changes in
the quiescent point of a transistor due variations in temperature, base-toemitter voltage and common emitter current gain.
Example 12.3
The self-bias circuit of Figure 12.6 has the following element values:
. K , RC = 6.8 K , βF varies from
RB1 = 50 K , RB 2 = 10 K , RE = 12
150 to 200 and VCC is 10 ± 0.05 V. I CBO is 1 µA at 25 0C. Calculate the
collector current at 25 oC and plot the change in collector current for
temperatures between 25 and 100 oC. Assume V BE and βF at 25 oC are 0.7 V
and 150, respectively.
© 1999 CRC Press LLC
Solution
Equations (12.25), (12.26), and (12.30) can be used to calculate the collector
current. At each temperature, the stability factors are calculated using
Equations (12.37), (12.39), (12,48) and (12.49). The changes in V BE and
I CBO with temperature are obtained using Equations (12.32) and (12.33),
respectively. The change in I C for each temperature is calculated using
Equation (12.36).
MATLAB Script:
% Bias stability
%
rb1=50e3; rb2=10e3; re=1.2e3; rc=6.8e3;
vcc=10; vbe=0.7; icbo25=1e-6; beta=(150+200)/2;
vbb=vcc*rb2/(rb1+rb2);
rb=rb1*rb2/(rb1+rb2);
ic=beta*(vbb-vbe)/(rb+(beta+1)*re);
%stability factors are calculated
svbe=-beta/(rb+(beta+1)*re);
alpha=beta/(beta+1);
svcc=1/(rc + (re/alpha));
svicbo=(rb+re)/(re+(rb+re)/alpha);
sbeta=((rb+re)*(vbb-vbe+icbo25*(rb+re))/(rb+re+beta*re)^2);
% Calculate changes in Ic for various temperatures
t=25:1:100;
len_t = length(t);
dbeta = 50; dvcc=0.1;
for i=1:len_t
dvbe(i)= -2e-3*(t(i)-25);
dicbo(i)=icbo25*(2^((t(i)-25)/10)-1);
dic(i)=svbe*dvbe(i)+svcc*dvcc...
+svicbo+dicbo(i)+sbeta*dbeta;
end
plot(t,dicbo)
title('Change in collector current vs. temperature')
xlabel('Temperature, degree C')
ylabel('Change in collector current, A')
Figure 12.9 shows
© 1999 CRC Press LLC
I C versus temperature.
Figure 12.9
12.3
I C versus Temperature
INTEGRATED CIRCUIT BIASING
Biasing schemes for discrete electronic circuits are not suitable for integrated
circuits (IC) because of the large number of resistors and the large coupling
and bypass capacitor required for biasing discrete electronic circuits. It is
uneconomical to fabricate IC resistors since they take a disproportionately
large area on an IC chip. In addition, it is almost impossible to fabricate IC
inductors. Biasing of ICs is done using mostly transistors that are connected to
create constant current sources. Examples of integrated circuit biasing
schemes are discussed in this section.
© 1999 CRC Press LLC
12.3.1
Simple current mirror
A simple current mirror is shown in Figure 12.10. The current mirror consists
of two matched transistors Q1 and Q2 with their bases and emitters connected.
The transistor Q1 is connected as a diode by shorting the base to its collector.
VCC
IO
IR
RC
IC1
Q1
Q2
IB1
IB2
Figure 12.10 Simple Current Mirror
From Figure 12.10, we observe that
IR =
VCC − VBE
RC
(12.50)
Using KCL, we get
I R = I C1 + I B1 + I B 2
= I E1 + I B2
But
I B2 =
© 1999 CRC Press LLC
I E2
β +1
(12.51)
Assuming matched transistors
I B1 ≅ I B 2
I E1 ≅ I E 2
(12.52)
From Equations (12.51) and (12.52), we get
I R = I E1 +
I E2
1
≅ I E 2 1 +
=
β +1
β + 1
and
I O = I C 2 = βI B 2 =
β + 2
β + 1I E2
(12.53)
βI E 2
β +1
Therefore
β β + 1
β
IO =
IR =
I
β +2 R
β + 1 β + 2
(12.54)
I O ≅ I R if β >> 1
(12.55)
Equation (15.55) is true provided Q2 is in the active mode. In the latter mode
of transistor operation, the device Q2 behaves as a current source. For Q2 to
be in the active mode, the following relation should be satisfied
VCE 2 > VCEsat
12.3.2
Wilson current source
The Wilson current source, shown in Figure 12.11, achieves high output
resistance and an output current that is less dependent on transistor βF . To
obtain an expression for the output current, we assume that all three transistors
are identical. Thus
© 1999 CRC Press LLC
I C1 = I C 2
VBE 1 = VBE 2
β F 1 = βF 2 = βF 3 = βF
(12.56)
VCC
IR
RC
IO
Q3
IB3
IE3
IC1
IE2
Q1
Q2
IB1
IB2
Figure 12.11 Wilson Current Source
Using KCL at the collector of transistor Q3 , we get
I C1 = I R − I B 3 = I R −
IO
βF
therefore,
I O = βF ( I R − I C1 )
Using KCL at the emitter of Q3 , we obtain
I E 3 = I C 2 + I B1 + I B 2 = I C 1 + 2 I B1
© 1999 CRC Press LLC
(12.57)
2
= I C1 1 +
βF
But
I0 = αF I E3 =
βF
I
βF + 1 E 3
(12.58)
(12.59)
Substituting Equation (12.58) into (12.59), we have
β
2
I 0 = F 1 + I C1
βF
βF + 1
(12.60)
Simplifying Equation (12.60), we get
β + 1
I
I C1 = F
βF + 2 0
(12.61)
Combining Equations (12.57) and (12.61), we obtain
β + 1
I
I 0 = βF I R − F
βF + 2 0
(12.62)
Simplifying Equation (12.62), we get
βF2 + 2 βF
I
I0 = 2
β F + 2 βF + 2 R
= 1 −
2
I
β + 2 βF + 2 R
2
F
For reasonable values of
βF
2
2
<< 1
βF + 2 βF + 2
© 1999 CRC Press LLC
(12.63)
and Equation (12.63) becomes
I0 ≅ I R
Thus,
β has little effect on the output current, and
IR =
VCC − V BE 3 − V BE 1
RC
(12.64)
Example 12.4
For Figures 12.10 and 12.11, what are the percentage difference between the
reference and output currents for the βF from 40 to 200. Assume that for
both figures,
VCC = 10 V , RC = 50 KΩ and VBE = 0.7 V .
Solution
I R and Equation (12.53) to find I 0 of
the simple current mirror. Similarly, we use Equation (12.64) to find I R and
Equation (12.63) to calculate I 0 of the Wilson current source.
We use Equation (12.50) to calculate
MATLAB Script
% Integrated circuit Biasing
vcc=10; rc=50e3; vbe=0.7;
beta =40:5:200; ir1=(vcc-vbe)/rc;
ir2=(vcc-2*vbe)/rc; m=length(beta);
for i=1:m
io1(i) = beta(i)*ir1/(beta(i) + 2);
pd1(i)=abs((io1(i)-ir1)*100/ir1);
io2(i)=(beta(i)^2+2*beta(i))/(beta(i)^2+2*beta(i)+2);
pd2(i)=abs((io2(i)*ir2-ir2)*100/ir2);
end
subplot(211), plot(beta,pd1)
%title('error for simple current mirror')
xlabel('Transistor beta')
ylabel('Percentage error')
text(90,5,'Error for simple current mirror')
subplot(212),plot(beta,pd2)
© 1999 CRC Press LLC
%title('Error for Wilson current mirror')
xlabel('Transistor beta')
ylabel('Percentage error')
text(90, 0.13, 'Error for Wilson current source')
Figure 12.12 shows the percentage errors obtained for the simple current
mirror and Wilson current source.
Figure 12.12 Percentage Error between Reference and Output
Currents for Simple Current Mirror and Wilson
Current Source
12.4
FREQUENCY RESPONSE OF COMMON EMITTER
AMPLIFIER
The common-emitter amplifier, shown in Figure 12.13, is capable of
generating a relatively high current and voltage gains. The input resistance is
medium and is essentially independent of the load resistance RL . The output
resistance is relatively high and is essentially independent of the source
resistance.
© 1999 CRC Press LLC
CC1 , couples the source voltage v S to the biasing
network. Coupling capacitor CC 2 connects the collector resistance RC to
the load RL . The bypass capacitance C E is used to increase the midband
gain, since it effectively short circuits the emitter resistance R E at midband
frequencies. The resistance R E is needed for bias stability. The external
capacitors CC1 , CC 2 , C E will influence the low frequency response of the
The coupling capacitor,
common emitter amplifier. The internal capacitances of the transistor will
influence the high frequency cut-off. The overall gain of the common-emitter
amplifier can be written as
A( s) =
Am s 2 (s + wz )
(s + w )(s + w )(s + w )(1 + s w )
L1
L2
L3
(12.65)
H
where
is the midband gain.
AM
is the frequency of the dominant high frequency pole
wH
w L1 , w L 2 , w L 3 are low frequency poles introduced by the
coupling and bypass capacitors
is the zero introduced by the bypass capacitor.
wZ
VCC
RC
RB1
Rs
Vs
CC2
+
CC1
RL
RB2
RE
Vo
CE
-
Figure 12.13 Common Emitter Amplifier
© 1999 CRC Press LLC
The midband gain is obtained by short circuiting all the external capacitors and
open circuiting the internal capacitors. Figure 12.14 shows the equivalent for
calculating the midband gain.
RS
Ib
Is
Vs
RB
r
Beta*I B
+
rce
π
RC
RL
Vo
-
Figure 12.14 Equivalent Circuit for Calculating Midband Gain
From Figure 12.14, the midband gain,
Am =
[
]
Am , is
RB
VO
= − β rCE RC R L
VS
RB + rπ
1
RS + RB rπ
[
It can be shown that the low frequency poles,
by the following equations
τ1 =
where
1
= CC1 RIN
w L1
[
RIN = RS + RB rπ
[
]
(12.66)
w L1 , w L 2 , w L 3 , can be obtained
(12.67)
]
(12.68)
(
τ2 =
1
= CC 2 R L + RC rce
wL2
τ3 =
1
= C E R E'
w L3
)]
(12.69)
and
where
© 1999 CRC Press LLC
(12.70)
r
RB RS
RE' = RE π +
βF + 1 βF + 1
(12.71)
and the zero
wZ =
1
RE C E
(12.72)
Normally, wZ < w L 3 and the low frequency cut-off w L is larger than the
largest pole frequency. The low frequency cut-off can be approximated as
wL ≅
(w ) + (w ) + (w )
2
2
L1
L2
2
(12.73)
L3
The high frequency equivalent circuit of the common-emitter amplifier is
shown in Figure 12.15.
Rs
Vs
B
rx
RB
cµ
B'
V
r
π
Figure 12.15
C
π
C
π
gm V
rce
Rc
π
RL
Vo
Equivalent Circuit of CE Amplifier at High
Frequencies
Cµ is the collector-base capacitance, Cπ is the emitter to
base capacitance, rX is the resistance of silicon material of the base region
In Figure 12.15,
between the base terminal B and an internal or intrinsic base terminal B’.
Using the Miller Theorem, it can be shown that the 3-dB frequency at high
frequencies is approximately given as
( [
(
w H −1 = rπ rx + RB RS
where
[
(
)])C
CT = Cπ + Cµ 1 + g m R L RC
and
© 1999 CRC Press LLC
T
)]
(12.74)
(12.75)
gm =
IC
VT
(12.76)
In the following example, MATLAB is used to obtain the frequency response
of a common-emitter amplifier.
Example 12.5
For a CE amplifier shown in Figure 12.13,
β = 150, RL = 2 KΩ, RC = 4 KΩ, Cπ = 100 pF , Cµ = 5 pF , VCC = 10 V ,
. KΩ, CC1 = 2 µF , CC 2 = 4 µF , C E = 150 µF ,
rce = ro = 60 KΩ, RE = 15
RB1 = 60 KΩ, RB 2 = 40 KΩ, RS = 100Ω, rx = 10 Ω.
Use MATLAB to plot the magnitude response of the amplifier.
Solution
Using Equations (12.67), (12.69), (12.70) and (12.74) are used to calculate the
poles of Equation (12.65). The zero of the overall amplifier gain is calculated
using Equation (12.66). The MATLAB program is as follows:
MATLAB Script
%Frequency response of CE Amplifier
rc=4e3; rb1=60e3; rb2=40e3; rs=100; rce=60e3;
re=1.5e3; rl=2e3; beta=150; vcc=10; vt=26e-3; vbe =0.7;
cc1=2e-6; cc2=4e-6; ce=150e-6;, rx=10; cpi=100e-12;
cmu=5e-12;
% Ic is calculated
rb = (rb1 * rb2)/(rb1 + rb2);
vbb = vcc * rb2/(rb1 + rb2);
icq = beta * (vbb - vbe)/(rb + (beta + 1)*re);
% Calculation of low frequency poles
% using equations (12.67), (12.69) and (12.70)
rpi=beta * vt/icq;
rb_rpi=rpi * rb/(rpi + rb);
rin=rs + rb_rpi;
wl1=1/(rin * cc1);
rc_rce=rc * rce/(rc + rce);
© 1999 CRC Press LLC
wl2=1/(cc2 * (rl + rc_rce));
rb_rs=rb * rs/(rb + rs);
rx1=(rpi + rb_rs)/(beta + 1);
re_prime=re * rx1/(re + rx1);
wl3=1/(re_prime * ce);
% Calculate the low frequency zero using equation (12.72)
wz = 1/(re*ce);
% Calculate the high frequency pole using equation (12.74)
gm = icq/vt;
rbrs_prx = ( rb * rs/(rb + rs)) + rx;
rt = (rpi * rbrs_prx)/(rpi + rbrs_prx);
rl_rc = rl * rc/(rl + rc);
ct = cpi + cmu * (1 + gm * rl_rc);
wh = 1/(ct * rt);
% Midband gain is calculated
rcercrl = rce * rl_rc/(rce + rl_rc);
am = -beta * rcercrl * (rb/(rb + rpi)) * (1/(rin));
% Frequency response calculation using equation (12.65)
a4 = 1; a3 = wl1 + wl2 + wl3 + wh;
a2 = wl1*wl2 + wl1*wl3 + wl2*wl3 + wl1*wh + wl2*wh + wl3*wh;
a1 = wl1*wl2*wl3 +wl1*wl2*wh + wl1*wl3*wh + wl2*wl3*wh;
a0 = wl1*wl2*wl3*wh;
den=[a4 a3 a2 a1 a0];
b3 = am*wh;
b2 = b3*wz; b1 =0; b0 = 0;
num = [b3 b2 b1 b0];
w = logspace(1,10);
h = freqs(num,den,w);
mag = 20*log10(abs(h));
f = w/(2*pi);
% Plot the frequency response
semilogx(f,mag,'w')
title('Magnitude Response')
xlabel('Frequency, Hz')
ylabel('Gain, dB')
axis([1, 1.0e10, 0, 45])
The frequency response is shown in Figure 12.16.
© 1999 CRC Press LLC
Figure 12.16 Frequency Response of a CE Amplifier
12.5
MOSFET CHARACTERISTICS
Metal-oxide-semiconductor field effect transistor (MOSFET) is a four-terminal
device. The terminals of the device are the gate, source, drain, and substrate.
There are two types of mosfets: the enhancement type and the depletion type.
In the enhancement type MOSFET, the channel between the source and drain
has to be induced by applying a voltage on the gate. In the depletion type
mosfet, the structure of the device is such that there exists a channel between
the source and drain. Because of the oxide insulation between the gate and the
channel, mosfets have high input resistance. The electronic symbol of a
mosfet is shown in Figure 12.19.
© 1999 CRC Press LLC
D
G
S
G
S
(a)
D
(b)
Figure 12.17 Circuit Symbol of (a) N-channel and
(b) P-channel MOSFETs
Mosfets can be operated in three modes: cut-off, triode, and saturation
regions. Because the enhancement mode mosfet is widely used, the
presentation in this section will be done using an enhancement-type mosfet. In
the latter device, the channel between the drain and source has to be induced
by applying a voltage between the gate and source. The voltage needed to
create the channel is called the threshold voltage, VT . For an n-channel
enhancement-type mosfet ,
negative.
VT is positive and for a p-channel device it is
Cut-off Region
For an n-channel mosfet, if the gate-source voltage VGS satisfies the condition
VGS < VT
(12.77)
then the device is cut-off. This implies that the drain current is zero for all
values of the drain-to-source voltage.
Triode Region
When VGS > VT and V DS is small, the mosfet will be in the triode region. In
the latter region, the device behaves as a non-linear voltage-controlled
resistance. The I-V characteristics are given by
© 1999 CRC Press LLC
[
I D = k n 2(VGS − VT )VDS − V DS 2
]
(12.78)
provided
VDS ≤ VGS − VT
where
kn =
and
µn
ε
ε ox
t ox
L
W
(12.79)
µn εεox W µn Cox
=
2t ox L
2
W
L
(12.80)
is surface mobility of electrons
is permittivity of free space ( 8.85 E-14 F/cm)
is dielectric constant of SiO2
is thickness of the oxide
is length of the channel
is width of the channel
Saturation Region
Mosfets can operate in the saturation region. A mosfet will be in saturation
provided
VDS ≥ VGS − VT
(12.81)
and I-V characteristics are given as
I D = k n (VGS − VT )
2
(12.82)
The dividing locus between the triode and saturation regions is obtained by
substituting
VDS = VGS − VT
into either Equation (12.78) or (12.82), so we get
© 1999 CRC Press LLC
(12.83)
I D = k nV DS 2
(12.84)
In the following example, I-V characteristics and the locus that separates triode
and saturation regions are obtained using MATLAB.
Example 12.6
For an n-channel enhancement-type MOSFET with
k n = 1 mA / V 2 and
. V , use MATLAB to sketch the I-V characteristics for
V = 15
VGS = 4, 6, 8 V and for VDS between 0 and 12 V .
Solution
MATLAB Script
% I-V characteristics of mosfet
%
kn=1e-3; vt=1.5;
vds=0:0.5:12;
vgs=4:2:8;
m=length(vds);
n=length(vgs);
for i=1:n
for j=1:m
if vgs(i) < vt
cur(i,j)=0;
elseif vds(j) >= (vgs(i) - vt)
cur(i,j)=kn * (vgs(i) - vt)^2;
elseif vds(j) < (vgs(i) - vt)
cur(i,j)= kn*(2*(vgs(i)-vt)*vds(j) - vds(j)^2);
end
end
end
plot(vds,cur(1,:),'w',vds,cur(2,:),'w',vds,cur(3,:),'w')
xlabel('Vds, V')
ylabel('Drain Current,A')
title('I-V Characteristics of a MOSFET')
text(6, 0.009, 'Vgs = 4 V')
© 1999 CRC Press LLC
text(6, 0.023, 'Vgs = 6 V')
text(6, 0.045, 'Vgs = 8 V')
Figure 12.18 shows the I-V characteristics.
Figure 12.18 I-V Characteristics of N-channel Enhancement-type
Mosfet
12.6
BIASING OF MOSFET CIRCUITS
A popular circuit for biasing discrete mosfet amplifiers is shown in Figure
12.19. The resistances RG1 and RG 2 will define the gate voltage. The
resistance
© 1999 CRC Press LLC
RS improves operating point stability.
VDD
RD
RG1
ID
VG
VDS
IS
RG2
RS
Figure 12.19 Simple Biasing Circuit for Enhancement-type NMOS
Because of the insulated gate, the current that passes through the gate of the
MOSFET is negligible. The gate voltage is given as
VG =
RG1
V
RG1 + RG 2 DD
The gate-source voltage
(12.85)
VGS is
VGS = VG − I S RS
(12.86)
For conduction of the MOSFET, the gate-source voltage
greater than the threshold voltage of the mosfet,
Equation (12.86) becomes
VGS = VG − I D RS
© 1999 CRC Press LLC
VT .
VGS should be
Since I D = I S ,
(12.87)
The drain-source voltage is obtained by using KVL for the drain-source circuit
VDS = VDD − I D RD − I S RS
= VDD − I D ( R D − RS )
(12.88)
For proper operation of the bias circuit,
VGS > VT
(12.89)
When Equation (12.89) is satisfied, the MOSFET can either operate in the
triode or saturation region. To obtain the drain current, it is initially assumed
that the device is in saturation and Equation (12.82) is used to calculate I D .
Equation (12.81) is then used to confirm the assumed region of operation. If
Equation (12.82) is not satisfied, then Equation (12.78) is used to calculate I D .
The method is illustrated by the following example.
Example 12.7
VT
= 2 V, k n = 0.5 mA/V2, V DD = 9V,
= 10 MΩ , RS = R D = 10 KΩ . Find I D and VDS .
For Figure 12.19,
RG1 = RG 2
Solution
Substituting Equation (12.86) into Equation (12.82), we have
(
I = k n Vg − I D RD − VT
)
2
(12.90)
Simplifying Equation (12.90), we have
[
]
(
0 = k n R D2 I D2 − 1 + 2(Vg − VT ) R D I D + k V g − VT
)
2
(12.91)
The above quadratic equation is solved to obtain I D . Two solutions of I D
are obtained. However, only one is sensible and possible. The possible one is
© 1999 CRC Press LLC
the one that will make
VGS > VT . With the possible value of I D obtained,
VDS is calculated using Equation (12.88). It is then verified whether
VDS > VGS − VT
The above condition ensures saturation of the device. If the device is not in
saturation, then substituting Equation (12.86) into Equation (12.78), we get
[
ID = kn 2(Vg − IDRD −VT )(VDD −(RD + RS )ID) −(VDD −(RD + RS )ID)
2
]
(12.92)
Simplifying Equation (12.92), we get the quadratic equation
[
]
0 = ID2 (RS + RD )2 + 2RD (RD + RS )
+ I D 2VDD (RD + RS ) − 2VDD RD − 2(Vg −VT )(RD + RS ) − 1k
n
2
+ 2(Vg −VT )VDD −VDD
Two roots are obtained by solving Equation (12.93).
possible root is the one that will make
VGS > VT
The MATLAB program for finding
I D is shown below.
MATLAB Script
%
% Analysis of MOSFET bias circuit
%
diary ex12_7.dat
diary on
vt=2; kn=0.5e-3; vdd=9;
rg1=10e6; rg2=10e6; rs=10e3; rd=10e3;
vg=vdd * rg2/(rg1 + rg2);
% Id is calculated assuming device is in saturation
a1=kn*(rd^2);
a2=-(1 + 2*(vg - vt)*rd * kn);
© 1999 CRC Press LLC
(12.93)
The sensible and
a3=kn * (vg - vt)^2;
p1=[a1,a2,a3];
r1=roots(p1);
% check for the sensible value of the drain current
vgs = vg - rs * r1(1);
if vgs > vt
id = r1(1);
else
id = r1(2);
% check for sensible value of the drain current
vgs = vg - rs*r2(1);
if vgs > vt
id = r2(1);
else
id=r2(2);
end
vds=vdd - (rs + rd)*id;
end
% print out results
fprintf('Drain current is %7.3e Amperes\n',id)
fprintf('Drain-source voltage is %7.3e Volts\n', vds)
The results are
Drain current is 1.886e-004 Amperes
Drain-source voltage is 5.228e+000 Volts
The circuit shown in Figure 12.20 is a mosfet transistor with the drain
connected to the gate. The circuit is normally referred to as diode-connected
enhancement transistor.
From Equation (12.88), the MOSFET is in saturation provided
VDS > VGS − VT
i.e.,
V DS − VGS > −VT or V DS + VSG > −VT
or
© 1999 CRC Press LLC
V DG > −VT
(12.94)
D
ID
G
VDS
S
Figure 12.20 Diode-connected Enhancement Type MOSFET
Since V DG = 0 and
saturation and
VT is positive for n-channel MOSFET, the device is in
i D = k n (VGS − VT )
But if VGS
2
(12.95)
= VDS , Equation (12.101) becomes
i D = k n (VDS − VT )
2
The diode-connected enhancement mosfet can also be used to generate dc
currents for nMOS and CMOS analog integrated circuits. A circuit for
generating dc currents that are constant multiples of a reference current is
shown in Figure 12.21. It is a MOSFET version of current mirror circuits
discussed in Section 12.3.
Assuming the threshold voltages of the transistors of Figure 12.21 are the
same, then since transistor T1 is in saturation,
I REF = k1 (VGS 1 − VT )
2
Since transistors T1 and T2 are connected in parallel, we get
© 1999 CRC Press LLC
(12.96)
VGS 1 = VGS 2 = VGS
and
(12.97)
I 0 = k 2 (VGS 2 − VT )
I 0 = k 2 (VGS − VT )
2
2
IREF
(12.98)
Io
Vo
T1
T2
Figure 12.21 Basic MOSFET Current Mirror
Combining Equations (12.96) and (12.98), the current
k2
I 0 = I REF
k1
(12.99)
and using Equation (12.74), Equation (12.99) becomes
( )
( )
W
L 2
I 0 = I REF
W
L 1
© 1999 CRC Press LLC
(12.100)
Thus, I 0 will be a multiple of I REF , and the scaling constant is determined by
the device geometry. In practice, because of the finite output resistance of
transistor T2, I 0 will be a function of the output voltage v 0 .
Example 12.8
. MΩ, L1 = L2 = 6 µm,
R1 = 15
W1 = 12 µm, W2 = 18 µm, VT = 2.0 V and VDD = 5 V. Find the output
current I D1 , VGS 1 , I 0 and R2 . Assume that V0 = 2.5 V, µCOX = 30
µA / V 2 . Neglect channel length modulation.
For the circuit shown in Figure 12.22,
VDD
VDD
R1
Io
R2
Vo
T1
T2
Figure 12.22 Circuit for Example 12.8
Solution
Since T1 is in saturation,
I D1 = k n1 (VGS − VT ) = k n1 (V DS − VT ) 2
(12.101)
VDS = VDD − I D1 R1
(12.102)
2
Substituting Equation (12.100) into (12.99), we get
© 1999 CRC Press LLC
I D1 = k n1 (VDD − VT − R1 I D1 )
2
I D1
2
= (VDD − VT ) − 2(VDD − VT ) R1 I D1 + R12 I D2 1
k n1
1
2
0 = R12 I D2 1 − 2(VDD − VT ) R1 +
I D1 + (VDD − VT )
k n1
(12.103)
The above quadratic equation will have two solutions, but only one of the
solution of I D1 will be valid. The valid solution will result in VGS > VT .
Using equation (12.100), we obtain
( )
( )
W
L 2
I 0 = I D1
W
L 1
and
R=
5 − V0
I0
The MATLAB program is as follows:
MATLAB Script
%
% Current mirror
%
diary ex12_8.dat
diary on
ucox = 30e-6; l1 = 6e-6; l2 = 6e-6;
w1 = 12e-6; w2=18e-6;
r1=1.5e6; vt=2.0; vdd=5; vout=2.5;
% roots of quadratic equation(12.103) is obtained
kn = ucox * w1/(2 * l1);
a1 = r1^2;
a2 = -2*(vdd - vt)*r1 - (1/kn);
© 1999 CRC Press LLC
(12.104)
(12.105)
a3 = (vdd - vt)^2;
p = [a1,a2,a3];
i = roots(p);
% check for realistic value of drain current
vgs=vdd - r1*i(1);
if vgs > vt
id1 = i(1);
else
id1 = i(2);
end
% output current is calculated from equation(12.100)
% r2 is obtained using equation (12.105)
iout = id1*w2*l1/(w1 * l2);
r2=(vdd - vout)/iout;
% print results
fprintf('Gate-source Voltage of T1 is %8.3e Volts\n',vgs)
fprintf('Drain Current of T1 is %8.3e Ampers\n', id1)
fprintf('Drain Current Io is %8.3e Ampers\n', iout)
fprintf('Resistance R2 is %8.3e Ohms\n', r2)
The results are
Gate-source Voltage of T1 is 1.730e+000 Volts
Drain Current of T1 is 1.835e-006 Ampers
Drain Current Io is 2.753e-006 Ampers
Resistance R2 is 9.082e+005 Ohms
12.7
FREQUENCY RESPONSE OF COMMON-SOURCE
AMPLIFIER
The common-source amplifier has characteristics similar to those of the
common-emitter amplifier discussed in Section 12.4. However, the commonsource amplifier has higher input resistance than that of the common-emitter
amplifier. The circuit for the common source amplifier is shown in Figure
12.23.
© 1999 CRC Press LLC
VDD
RD
RG1
RI
CC2
+
CC1
Vo
Vs
R1
RG2
CS
RS
-
Figure 12.23 Common-Source Amplifier
The external capacitors CC1 , CC 2 and CS will influence the low frequency
response. The internal capacitances of the FET will affect the high frequency
response of the amplifier. The overall gain of the common-source amplifier
can be written in a form similar to Equation (12.65).
The midband gain, Am , is obtained from the midband equivalent circuit of the
common-source amplifier. This is shown in Figure 12.24. The equivalent
circuit is obtained by short-circuiting all the external capacitors and opencircuiting all the internal capacitances of the FET.
RI
+
Vs
Vgs
RG
rds
RD
RL
Vo
gmVgs
-
Figure 12.24 Midband Equivalent Circuit of Common-Source
Amplifier
Using voltage division,
v gs =
RG
v
R I + RG S
From Ohm’s Law,
© 1999 CRC Press LLC
(12.106)
(
v 0 = − g m v gs rds R D R L
)
(12.107)
Substituting Equation (12.106) into (12.107), we obtain the midband gain as
Am =
(
RG
v0
r R R
= − gm
vs
RG + R I ds D L
)
(12.108)
At low frequencies, the small signal equivalent circuit of the common-source
amplifier is shown in Figure 12.25.
CC1
RI
CC2
+
Vgs
+
+
gmVgs
rds
RG
VS
VO
RS
-
Cs
RD
RL
-
Figure 12.25 Equivalent Circuit for Obtaining the Poles at Low
Frequencies of Common-source Amplifier
It can be shown that the low frequency poles due to
written as
CC1 and CC 2 can be
τ1 =
1
≅ CC 1 ( R g + R I )
w L1
(12.109)
τ2 =
1
≅ CC 2 ( RL + RD rds )
wL 2
(12.110)
Assuming rd is very large, the pole due to the bypass capacitance
shown to be
τ3 =
and the zero of
© 1999 CRC Press LLC
RS
1
≅ CS
w L3
1 + g m RS
CS is
CS can be
(12.111)
wZ =
1
RS CS
(12.112)
The 3-dB frequency at the low frequency can be approximated as
wL ≅
(w ) + (w ) + (w )
2
2
L1
L2
2
(12.113)
L3
For a single stage common-source amplifier, the source bypass capacitor is
usually the determining factor in establishing the low 3-dB frequency.
The high frequency equivalent circuit of a common-source amplifier is shown
in Figure 12.26. In the figure, the internal capacitances of the FET, C gs , C gd
and Cds are shown. The external capacitors of the common of commonsource amplifier are short-circuited at high frequencies.
Cgd
RI
+
+
VS
RG
Cgs
gm Vgs
Cds
rds
RD
RL
VO
-
Figure 12.26 High Frequency Equivalent Circuit of Commonsource Amplifier
Using the Miller theorem, Figure 12.26 can be simplified. This is shown in
Figure 12.27.
The voltage gain at high frequencies is
AV =
where
and
© 1999 CRC Press LLC
RG
v0
gm RL'
≅ −
'
vs
R
R
+
G
I 1 + s( RG RI )C1 (1 + sRL C2 )
(
)
(12.114)
C1 = C gs + C gd (1 + g m R L' )
(12.115)
C2 = Cds + C gd
(12.116)
RI
+
+
VS
Cgs
RG
Cds
-
Cgd
RL'
VO
gmVgs
Cgs (1+gmR'L)
Figure 12.27 Simplified High Frequency Equivalent Circuit for
Common-source Amplifier
The high frequency poles are
wH1 =
wH 2 =
(
1
C1 RG RI
(
)
1
C2 R L RD rds
(12.117)
)
(12.118)
The approximate high frequency cut-off is
wH =
1
2
1
1
+
wH 2
wH1
2
(12.119)
In the following example, MATLAB is used to obtain the midband gain, cutoff frequencies and bandwidth of a common-source amplifier.
© 1999 CRC Press LLC
Example 12.9
For the common-source amplifier, shown in Figure 12.23,
CC1 = CC 2 = 1µF , CS = 50 µF . The FET parameters are
Cgd = Cds = 1 pF , Cgs = 10 pF , g m = 10 mA / V , rds = 50 KΩ.
RD = 8 KΩ, RL = 10 KΩ, RS = 2 KΩ, RI = 50 Ω,
RG1 = 5 MΩ, RG 2 = 5 MΩ.
Determine (a) midband gain, (b) the low frequency cut-off, (c) high
frequency cut-off, and (d) bandwidth of the amplifier.
Solution
MATLAB Script
%
% common-source amplifier
%
diary ex12_9.dat
diary on
rg1=5e6; rg2=5e6; rd=8e3; rl=10e3;
ri=50; rs=2e3; rds=50e3;
cc1=1e-6; cc2=1e-6; cs=50e-6;
gm=10e-3; cgs=10e-12; cgd=1e-12; cds=1e-12;
% Calculate midband gain using equation (12.108)
a = (1/rds) + (1/rd) + (1/rl);
rlprime = 1/a;
rg = rg1*rg2/(rg1 + rg2);
gain_mb = -gm*rg*rlprime/(ri + rg);
% Calculate Low cut-off frequency using equation (12.113)
t1 = cc1*(rg + ri);
wl1 = 1/t1;
rd_rds = (rd*rds)/(rd + rds);
t2 = cc2 * (rl + rd_rds);
wl2=1/t2;
t3=cs * rs/(1 + gm * rs);
wl3=1/t3;
wl=sqrt(wl1^2 + wl2^2 + wl3^2);
© 1999 CRC Press LLC
% Calculate high frequency cut-off using equations (12.115 to
12.119)
c1=cgs + cgd * (1 + gm * rlprime);
c2=cds + cgd;
rg_ri=rg * ri/(rg + ri);
wh1=1/(rg_ri * c1);
wh2=1/(rlprime * c2);
int_term = sqrt((1/wh1)^2 + (1/wh2)^2);
wh = 1/int_term;
bw = wh-wl;
% Print results
fprintf('Midband Gain is %8.3f\n', gain_mb)
fprintf('Low frequency cut-off is %8.3e\n', wl)
fprintf('High frequency cut-off is %8.3e\n', wh)
fprintf('Bandwidth is %8.3e Hz\n', bw)
The results are
Midband Gain is -40.816
Low frequency cut-off is 2.182e+002
High frequency cut-off is 1.168e+008
Bandwidth is 1.168e+008 Hz
SELECTED BIBLIOGRAPHY
© 1999 CRC Press LLC
1.
Geiger, R.L., Allen, P.E., and Strader, N.R., VLSI Design Techniques
for Analog and Digital Circuits, McGraw Hill Publishing Co., 1990.
2.
Sedra, A.S. and Smith, K.C., Microelectronics Circuits, 4th
Edition, Oxford University Press, 1997.
3.
Savant, C.J., Roden, M.S., and Carpenter, G.L., Electronic Circuit
Design: An Engineering Approach, Benjamin Cummings Publishing
Co., 1987.
4.
Ferris, C.D., Elements of Electronic Design, West Publishing Co.,
1995.
5.
Ghausi, M.S., Electronic Devices and Circuits: Discrete and
Integrated, Holt, Rinehart and Winston, 1985.
6.
Warner Jr., R.M. and Grung, B.L., Semiconductor Device
Electronics, Holt, Rinehart and Winston, 1991.
7.
Belanger, P.R., Adler, E.L. and Rumin, N.C., Introduction to
Circuits with Electronics: An Integrated Approach, Holt, Rinehart
and Winston, 1985.
8.
Wildlar R.J., Design techniques for monolithic operational amplifiers,
IEEE Journal of Solid State Circuits, SC-3, pp. 341 - 348, 1969.
EXERCISES
12.1
For the data provided in Example 12.2, Use MATLAB to sketch the
output characteristics for V BE = 0.3, 0.5, 0.7 V. Do not neglect the
effect of V AF on the collector current.
12.2
For the self-bias circuit, shown in Figure 12.6, the collector current
involving I CBO is given by Equation (12.47). Assuming that
RB1 = 75 KΩ , RB2 = 25 KΩ , RE = 1 KΩ , RC = 7.5 KΩ ,
βF = 100, and at 25o C, V BE = 0.6 V and I CBO = 0.01 µA,
determine the collector currents for temperatures between 25 oC and
85 oC. If R E is changed to 3 KΩ , what will be the value of I C ?
12.3
RB1 = 50 KΩ , RB2 = 40 KΩ , rS = 50 Ω ,
rX = 10 Ω , R L = 5 KΩ , RC = 5 KΩ , rce = 100 KΩ ,
CC1 = CC 2 = 2 µF , Cπ = 50 pF , Cµ = 2 pF , βF = 100, VCC
For Figure 12.13, if
= 10 V, explore the low frequency response for the following values
of R E : 0.1 KΩ, 1 KΩ, 5 KΩ. Calculate the high frequency cut-off
for
12.4
RE = 0.1 KΩ.
For the Widlar current source, shown in Figure P12.4, determine the
output current if RC = 40 KΩ , VCC = 10 V, V BE1 = 0.7 V and
R2 = 25 KΩ .
© 1999 CRC Press LLC
VC
IR
IO = IC2
RC
IC1
Q2
Q1
IB1
IB2
IE2
RE
Figure P12.4 Widlar Current Source
12.5
For the n-channel enhancement-type MOSFET with
k n = 2 mA / V 2 and VT = 1 V , write a MATLAB program to
plot the triode characteristics for VGS = 2 , 3, 4, 5 V when
VDS < 1 V .
VT = 1.5 V, k n = 0.5 mA/V2, VDD = 10V,
RG1 = 10 MΩ, RG 2 = 12 MΩ, and RD = 10 KΩ . Find I D
for the following values of RS : 2, 4, 6, 8 KΩ. Indicate the region
of operation for each value of RS .
12.6
For Figure 12.19,
12.7
For the common-source amplifier shown in Figure 12.23,
. KΩ, RS = 100 Ω,
RD = 10 KΩ, RL = 1 MΩ, RSB = 15
RG1 = 10 MΩ, RG 2 = 10 MΩ, CC1 = CC 2 = 2 µF ,
CS = 40 µF . The FET parameters are Cgs = 10 pF ,
. pF , g m = 5 mA / V , and
Cgd = Cds = 15
rds = 100 KΩ. Use MATLAB to plot the frequency response of
the amplifier.
© 1999 CRC Press LLC
